Got some info on Megas, apparently you can have as many pokemon hold MegaStones as you want in battle. The only restriction you have is that you can only MegaEvolve one per battle.
Wow so it appears we can actually have Mega Stones held by multiple Pokemon at a time, but we can still only have one Mega Evolution active at a time. That's at least better than the last rumor that said we could only have one member of the whole party holding a Mega Stone.