I would normally put a mega stone on only one Pokemon, but putting multiple stones on them could have its advantages, in the case of the one you want to mega-evolve fainting prematurely. Or if it's weak to most of the Pokemon you are battling. You could really go either way.
I would say, though, having multiple mega stones in-game is better than in competitive, because you can better afford to lose a Pokemon with revives in hand. In-game you can basically mega evolve whenever you want to, but in competitive you have to have good timing. Again, either way has its ups and downs Without focus sashes and all that, you can't stay alive, but then without multiple mega stones, if your mega faints you're sunk.
I'm not sure if I made much sense, so I'm hoping someone else who knows a bit more of what they're talking about will answer this in more detail. If I did, however, make sense I hope I helped.