The procedural standards for severing co-accused service members—that is, trying them separately instead of in a joint trial—are governed by the Rules for Courts-Martial. The general rule and preference in the military is that accused persons who are alleged to have participated in the same act or transaction should be tried together. This is for reasons of judicial economy and efficiency. However, this preference gives way when a joint trial would be unfair to one of the accused.
A military defense attorney can file a “motion for severance” with the military judge, asking for a separate trial for their client. The attorney must show that their client would be prejudiced by a joint trial. The most common ground for severance is when the co-accused have “antagonistic defenses.” This occurs when the core of one accused’s defense is to blame the other, making it impossible for a single panel to fairly assess both cases simultaneously.
Another common reason is when the government intends to introduce a confession from one co-accused that also implicates the other. Under the Bruton rule (from a Supreme Court case), this is a violation of the other accused’s Sixth Amendment right to confront the witnesses against them, as they cannot cross-examine their co-accused who will not testify. If the judge finds that a joint trial would result in clear prejudice that cannot be cured by instructions to the panel, they will grant the motion for severance and order separate trials.