>>58368121In traditional Pokemon battles, yes. In PLZA, its just a matter of charging the Mega meter which, theoretically can be done multiple times per battle.
It does beg the question of whether or not battles will last long enough for multiple rounds of Mega evolution, though. For the raid bosses, there are ways to keep an activated Mega gauge going for longer, so I suspect typical battles will rarely see 2 separate uses of Mega evolution.