At their joint trial, a jury found Patricia A. M.1 and her husband David guilty of ten counts of sexual assault and one count of incest with a child, all as party to a crime.2 The charges involved Patricia's eight-year-old son Jeremiah and Tony, an eleven-year-old neighbor boy. Patricia argues that the trial court erred by refusing to sever her trial from her husband's or, in the alternative, by failing to give the jury a limiting instruction, thus prejudicing her separate interest. In addition, Patricia raises two multiplicity issues.
We conclude that the joint trial was prejudicial because deunaging evidence, irrelevant as to Patricia, was admitted and the limiting instruction required by State v. DiMaggio, 49 Wis. 2d 565, 577, 182 N.W.2d 466, 473, cert. denied sub nom., Pipito v. Wisconsin, 404 U.S. 838 (1971), once severance is denied was not given. We therefore reverse on the severance issue and remand for a new trial. We are unpersuaded by Patricia's multiplicity arguments, however, and so affirm that determination.
Testimony at trial elicited the following facts. On August 25, 1989, Tony spent the night at Jeremiah's. The two boys were showering together when Patricia entered the bathroom, reached past the shower curtain, and fondled both boys' genitals for several minutes.
*730After their shower, the boys entered the living room on their way to their bedroom. The living room also served as Patricia's and David's bedroom. Patricia was sitting naked on an opened sofa bed; David was on a cot a few feet away. Patricia directed each boy to take turns engaging in various sex acts with her, both individually and simultaneously, while David watched. David did not get physically involved but occasionally verbally encouraged the boys' actions. The entire episode, bathroom to living room, took place over approximately one hour.
At trial, Jeremiah and Tony testified to all of the behavior alleged against Patricia. Jeremiah proved to be a reluctant witness, however, in testifying about any behavior involving David.3 Both David's and Patricia's theory of defense was that the boys fabricated the entire scenario, gaining their knowledge of sexual matters from pornographic movies viewed at Tony's birthday party some months before.
The state then called as a witness Dr. Paul Gochis, a physician on the Sexual Assault Program team in the emergency department at a local hospital. Dr. Gochis had examined Jeremiah when the investigation first began. Dr. Gochis testified over defense counsel's objection that Jeremiah related to him conduct alleged in the complaint as well as uncharged instances of sodomy by David. Dr. Gochis stated that Jeremiah demonstrated this conduct for him by using anatomically correct dolls. Dr. Gochis also testified that upon physically examining Jeremiah, he found anal scarring and loss of anal sphinc*731ter tone, and that such findings suggested repeated anal penetration. Dr. Gochis opined that this physical evidence was consistent with Jeremiah's statement that David had sodomized him. The medical report describing the physical evidence was admitted as "other acts" evidence under sec. 904.04(2), Stats. Jeremiah's alleged statements about David's acts were admitted as a prior inconsistent statement under sec. 908.01(4)(a)1, Stats.4 His alleged statements about Patricia's acts were admitted as a prior consistent statement under sec. 908.01(4) (a) 2.
Twice pretrial, Patricia unsuccessfully moved to sever her trial from David's, objecting to Dr. Gochis' testimony about evidence pertaining to the uncharged sexual abuse, asserting that it was relevant only to David. Subsequent motions for mistrial also were denied. Likewise, her sec. 809.30, Stats., motion seeking post-conviction relief on the issues of severance and admission of "other acts" evidence was denied.
SEVERANCE
Patricia first argues that admission of Dr. Gochis' medical testimony was relevant only to David and thus made the potential for jury confusion so great that severance was necessary to a fair trial. Specifically, she complains that admission of Dr. Gochis' testimony regarding anal contact could have been wrongly viewed by the jury as evidence of her guilt in the charged crimes. In the alternative, she argues that absent severance, the court should have cautioned the jury to disregard as to her the *732medical evidence relating to David. We agree that the court erred in not employing one of those options.
A trial court has the power to try defendants together when they Eire charged with the same offense Eirising out of the same transaction and provable by the same evidence. Section 971.12, Stats.; State v. Brown, 114 Wis. 2d 554, 559, 338 N.W.2d 857, 860 (Ct. App. 1983). Granting or denying severance is left to the sound discretion of the trial court. State v. Jennaro, 76 Wis. 2d 499, 505, 251 N.W.2d 800, 803 (1977). Absent an abuse of discretion, we will not disturb the trial court's ruling. Brown, 114 Wis. 2d at 559, 338 N.W.2d at 860. What constitutes an abuse of discretion depends upon the facts of each case. Id. Although a single trial may be desirable from the stEindpoint of economical or efficient criminal procedure, "the right of a defendant to a fair trial must be the overriding consideration." Id.
Severance generally is considered appropriate in "line of evidence" cases — cases where a body of evidence is relevant to the liability of only one of the co-defendants — because of the risk that the trier of fact may treat all of the evidence as evidence against both defendsmts. State v. Suits, 73 Wis. 2d 352, 361-62, 243 N.W.2d 206, 211 (1976); see also sec. 971.12(3), Stats. Our supreme court has defined the trial court's role in line of evidence cases. In State v. DiMaggio, 49 Wis. 2d 565, 182 N.W.2d 466, cert. denied sub nom., Pipito v. Wisconsin, 404 U.S. 838 (1971), the court held:
If it appears during the course of the trial that a good deal of evidence applicable to only one defendant is being developed, the trial [court] has an option. [It] may order a severance at that time or the court may elect to give the jury a cautionary instruction to the *733effect that evidence against one may not be treated as evidence against [both], simply because they are being tried together.
Id. at 577, 182 N.W.2d at 473 (citation omitted; emphasis added).
At oral argument, the state contended that the trial court's failure either to sever or to caution the jury was not error because, as a threshold matter, there did not develop a "good deal of evidence." Id. (emphasis added).5 Not only do we disagree, but this position is contrary to the state's own brief. In its brief, the state argued that the challenged evidence would have been admissible against Patricia in a separate trial. The state then urged this court to "assign substantial probative value to the medical evidence" because Jeremiah was a poor witness on the stand and because the medical evidence was "the only physical evidence corroborating Jeremiah's statements to Dr. Gochis.”
We fail to see how this evidence possesses "substantial probative value" yet does not constitute a "good deal" of evidence. We do not here decide if DiMaggio establishes that the trial court must determine as a threshold matter whether there exists a "good deal" of evidence. Whether or not it does, we conclude that a "good deal" of evidence existed here and agree with the state that this evidence possesses "substantial probative value," thus triggering the further requirement that the court either sever or issue an admonitory instruction.
*734The supreme court expressly noted that severance based on line of evidence grounds may not seem warranted prior to trial because at that stage it often is difficult for the trial court to ascertain "exactly what evidence [is] applicable to whom." Id. Furthermore, once a joint trial is underway, severance may prove to be a practical impossibility because of the time, resources and energy the parties and the court already have invested. If the trial court thus elects either before or during trial not to sever, the cautionary instruction option becomes critically important because realistically, it may remain the sole safeguard of a potentially prejudiced defendant's right to a fair trial.
Here, Patricia moved twice for severance, clearly stating the grounds. The trial court denied both motions, as was within its discretion to do. The challenged evidence was then presented at trial. When it became clear that a "good deal of evidence" pertaining only to David was developing, it became incumbent upon the court to instruct the jury as directed by DiMaggio.6
A defendant has a due process right to a fair trial. See Brown, 114 Wis. 2d at 559, 338 N.W.2d at 860. "[D]ue process is an exact synonym for fundamental fairness." In re D.M.D., 54 Wis. 2d 313, 318, 195 N.W.2d 594, 597 (1972). Fundamental fairness demands that a jury receive proper instructions which fully and fairly inform it of the law it is to apply. See State v. Hurd, 135 Wis. 2d 266, 275, 400 N.W.2d 42, 46 (Ct. App. 1986). *735The instructions also must assist the jury in analyzing the evidence. State v. Waalen, 125 Wis. 2d 272, 274, 371 N.W.2d 401, 402 (Ct. App. 1985).
The state concedes no limiting instruction was given here and that the trial court expressly found no evidence of a conspiracy between David and Patricia. Thus, the trial court should have assisted the jury by giving a limiting instruction. Such an instruction would have helped the jury analyze the evidence which pertained only to David. Fairness to Patricia and to the jurors deciding her fate demands that much.
DiMaggio notwithstanding, the state argues that even though severance was not granted and the limiting instruction not given, we still may affirm the trial court. Citing Suits, the state contends that the line of evidence must be more than prejudicial; it also must be wholly irrelevant or otherwise inadmissible against the complaining defendant. Suits, 73 Wis. 2d at 362, 243 N.W.2d at 212. In other words, the state asserts that if the evidence could have been admitted at Patricia's separate trial, she suffered no prejudice at her joint trial. See Jennaro, 76 Wis. 2d at 505, 251 N.W.2d at 803.
We are not persuaded. The evidence suggesting anal penetration is irrelevant as to Patricia. It relates to none of the charges against her. There was no suggestion by the trial court or any of the parties that the information contained in the medical report involved Patricia, and the trial court found that no conspiracy existed between David and Patricia. Furthermore, Dr. Gochis testified that Jeremiah reported anal contact only with David. We fail to see how "other acts" of David would have been admissible against Patricia at a separate trial.
*736Moreover, we do not read Suits as dispensing with the requirement that a cautionary instruction be given. There, the court expressly concluded that admitting the line of evidence did not prejudice the complaining co-defendant — in part because of the "timely warning of the trial court." Suits, 73 Wis. 2d at 363, 243 N.W.2d at 212.
We thus conclude that while the denial of severance may have been a proper exercise of discretion, the trial court ultimately erred. We reiterate that in a line of evidence case where the trial court has denied severance, the trial court is then bound to give — sua sponte, if necessary — a DiMaggio-type instruction to the effect that evidence against one defendant cannot be considered against the other simply because they are being tried together.
MULTIPLICITY
Next, Patricia claims that the charges against her are multiplicitous, thus violating the double jeopardy provisions of the state and federal constitutions. Patricia was convicted as party to a crime on ten counts: counts 1 and 2 alleged penis contact; counts 3 and 4 alleged fellatio; counts 5 and 6 alleged that she caused each boy to have contact with her breasts; counts 7 and 8 alleged sexual intercourse; and counts 9 and 10 alleged cunnilingus. She also was convicted on an eleventh count, incest with a child. Counts 1, 3, 5, 7, 9 and 11 pertained to Jeremiah; counts 2, 4, 6, 8 and 10 pertained to Tony.
First, Patricia asserts that the first-degree sexual assault charges — counts 3, 4, 5 and 6 — are multiplicitous because the acts of fellatio and breast contact, which allegedly occurred simultaneously, were part of the same general transaction or episode.
*737Multiplicity exists when a defendant is charged in more than one count for a single offense. State v. Hirsch, 140 Wis. 2d 468, 471, 410 N.W.2d 638, 639 (Ct. App. 1987). Whether charges are multiplicitous is a question of constitutional fact which we review de novo. Id. at 470-71, 410 N.W.2d at 639. To make that determination, we must ascertain whether the charges are identical in law and fact, and what the legislature intended regarding the allowable unit of prosecution under the statute. Id. at 471, 410 N.W.2d at 639-40. The overall test is one of fundamental fairness or prejudice to the defendant. Id. at 471-72, 410 N.W.2d at 640.
As to identity in law, counts 3 and 4 are identical in law because both allege fellatio with a child under the age of thirteen. Likewise, counts 5 and 6 are identical in law because both allege that Patricia caused a child under the age of thirteen to have contact with her breasts.
As to identity in fact, however, although counts 3 and 4 both allege fellatio, each involves a different boy, thus requiring proof of an additional fact. Similarly, counts 5 and 6 allege that the same act — breast contact — occurred with different boys. When different victims are involved, generally there is a corresponding number of distinct crimes. State v. Rabe, 96 Wis. 2d 48, 67, 291 N.W.2d 809, 818 (1980). Patricia urges us to depart from this general rule, contending that despite the existence of two victims, separate charges are inappropriate under Hirsch because the alleged acts were part of the same general transaction or episode. We disagree.
In Hirsch, the defendant was convicted of three counts of first-degree sexual assault for allegedly touching a five-year-old girl's vaginal area, then her anal area *738and then her vaginal area a second time. The court of appeals reversed, ruling the charges multiplicitous. Noting that the episode took no more than a few minutes with little, if any, lapse of time between the alleged acts, the court stated that it could not say that the touchings were not part of the same general transaction or episode, or that" 'the defendant had sufficient time for reflection between the assaultive acts to again commit himself.' " Hirsch, 140 Wis. 2d at 475, 410 N.W.2d at 641 (quoting Harrell v. State, 88 Wis. 2d 546, 560, 277 N.W.2d 462, 467 (Ct. App. 1979)). The court concluded that the alleged acts were "not 'so significantly different in fact that they may be properly denominated separate crimes.' " Id. at 474, 410 N.W.2d at 641 (quoting State v. Eisch, 96 Wis. 2d 25, 34, 291 N.W.2d 800, 805 (1980)).
Here, the complaint and testimony stated that one boy allegedly had contact with Patricia's breasts at the same time that Patricia was performing fellatio on the other boy. Those acts are significantly different enough to be considered separate crimes. Also, unlike the brief assault in Hirsch, here the activities spanned an hour, giving Patricia time to reflect upon her conduct.
The first prong of the multiplicity test satisfied, we next turn to whether the legislature intended separate punishments for committing the same violation of sec. 948.02(1), Stats., against two different child victims. Patricia offers no discussion on this point. The state, by contrast, persuasively argues that the intent to permit separate punishments is evident by the statute's placement in ch. 948, Stats., "Crimes Against Children." We agree that the legislature could not have intended for a defendant who sexually assaults numerous children in one continuous episode to be subjected to the same pun*739ishment as a defendant who assaults only one child. We therefore conclude that counts 3, 4, 5 and 6 are not multiplicitous.
Patricia's second multiplicity claim is that counts 1, 3, 5, 7 and 9 are lesser included offenses of count 11, incest with a child. Patricia did not raise this issue prior to her appeal, however. Having failed to do so, she did not preserve the issue for our review. See State v. Hartman, 145 Wis. 2d 1, 9-10, 426 N.W.2d 320, 323 (1988).
By the Court. — Judgment and order reversed and cause remanded.
We are masking the identity of the defendant to shield the victims of this sensitive crime.
David's conviction is not part of this appeal.
Jeremiah grew particularly noncommittal when the questioning turned to David's role — active or passive — in any sexual activity with him. Specifically in regard to anal contact or intercourse, Jeremiah either stated he did not remember or flatly denied that he and David ever engaged in that activity.
Jeremiah testified that he remembered making statements to Dr. Gochis but claimed not to remember what he told the doctor.
At oral argument the state attempted to minimize the evidence by pointing out that it constituted only a small portion of a 1300-page transcript. We see this, however, as a question of quality, not quantity. Brevity may be more than the soul of wit: a witness' unembellished "He's the man I saw" may constitute extremely critical evidence in the lengthiest of trials.
It is immaterial that Patricia did not request a limiting instruction because State v. DiMaggio, 49 Wis. 2d 565, 182 N.W.2d 466, cert. denied sub nom., Pipito v. Wisconsin, 404 U.S. 838 (1971), places no burden on a co-defendant to make such a request. Once the trial court denied Patricia's severance motions, it was duty-bound to follow up with a limiting instruction.