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Right now the time span between actually knowing which side/map you are on and the start of the match only allows for changing one merc due to the slow responsitivity of the UI. Most of the time that's not a problem, but every now and then you might want to switch more than one merc per match.
If you are really fast you can start switching a second merc but the timer runs out before your party gets updated for the current match. Your selection still gets carried over to the next lobby screen, though.