e.g. Me 410: .50 cal, crappy 20mm (not 50cal-like ballistics like e.g. Hispano 20mm), and MK103. 3 really different ballistics that you can't separate in the hangar -- you must select either 13mm+20mm or 7.9mm+20mm, and then add some 2x20mm / 4x20mm / 2x30mm / BK5 option, on top of the 13/7,9 option.
IIRC (I'll get burned for this if I'm wrong, but IIRC) the MK103 was normally used only with a pair of 20mm (only 2 triggers needed). In any case... It just doesn't make sense to be forced to fire two such different weapons together.
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Or maybe make the MK103 selectable like the BK5? But that's pretty selfish solution, that only applies to 410 instead of any other model that has more than 2 different-enough weapons. The more elegant solution is still 3d trigger, and opt-in so it doesn't bother anyone's habits.