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: