I'm guessing you don't quite know how it works. A pokemon needs to be holding it's respective mega stone to mega evolve (which is done during battle, and ends after battle) using a mega ring to activate it. The stone doesn't get used up, so while you can have more of the same pokemon, the only one that can mega evolve during battle is the one holding it.
Hope that helped!
"I'm in the bathroom with a tommy gun droppin' gangsta ****." - Celph Titled