Mont. Code Ann. § 25-7-203

If an issue is brought to trial by a jury while a jury is impaneled in another cause and not then discharged, the court may order a jury for the trial of that issue to be drawn out of the box containing the ballots then undrawn; but in any other case the ballots with the names of all the trial jurors returned and attending the court must be placed together in the same box before a jury is drawn therefrom.