I've been playing Pokemon X for a while now, and about a month ago I caught myself a Mewtwo (score!) after I caught it, I found a Mewtwonite X. I decided to put Mewtwo in my party, alongside my Delphox, Xerneas, Raichu, Talonflame and Blastoise, and that's when a guy challenged me to a battle. I thrashed him (of course) but I found it strange that after I Mega Evolved my Blastoise, I couldn't Mega Evolve my Mewtwo during the same battle... Anybody know why this is?
It's a built-in game mechanic. Per the wiki:
Once per battle, only a single Pokémon for each Trainer may Mega Evolve. Switching the Mega Evolved Pokémon out will not dissolve a Mega Evolution. A Mega Evolved Pokémon can only revert back to its original state upon fainting or the conclusion of the battle. If a Mega Evolved Pokémon faints, but is later revived, that Pokémon cannot obtain a Mega Evolved state again for the remaining duration of the battle.