>>14412564So far, our best guess is that you're only allowed to mega-evolve once per battle. At least that's how it works in-game.
I think most standard OU teams will be happy with one mega stone bearer, although I can really see some peopel opt for 2 in order to be able to react to different situations and opponents. IMO, it's a string point von Venusaur in sun teams e.g.: If you win the weather war, you mega-evolve your main fire sweeper and abuse saur's chlorophyll. If you lose it, you mega-evolve saur to battle rain or sand and fodder your now useless fire sweeper.
Either way, having multiple mega stone users in a team will be important for VGC, IMO, because you still want to have picking options.