[quote=@Sho Minazuki]
Mega Evolution requires a user's keystone, and the Pokemon to hold a Mega Stone.

You can have multiple that can turn Mega in your team, but only the one that has the Mega Stone can Mega Evolve.

The logic of it in the anime is that the evolution must also require a strong bond between the trainer and the Pokemon.
[/quote]

yea basically Mega evolution requires two stones, also the pokemon has to be trained to handle mega evolution 

*cough lucario cough cough* or you may end up with a rampage. and as Sho said you can have mutiple mega evolution pokemon in your team but only one can really turn mega due to you only having one keystone.

so yea