So this theory is about Mewtwonite X & Y and Mewtwo's reason to have 2 mega evolutions, this theory is also about Charizardite X & Y and Charizard's reason to have 2 mega evolutions. It's odd how Mewtwo and Charizard are the only two Pokémon to have two mega evolutions (so far). Doesn't it seem odd that if you were to have Pokémon Y (Yveltal) you would get mega Mewtwo Y and that if you had Pokémon X (Xerneas) you would get mega Mewtwo X. You can only get Mewtwo after capturing these respective Pokémon.
If you were to take both of them, (If a sequel came where you had to catch them both which I will refer to as Z) The reaction would end up with the full Mewtwonite. The theory is that Mewtwonite X & Y are incomplete halves of the full mega stone. Charizard is a similar case. Energy is radiating from Geosenge town, depending on your version of the game, the half of the full Charizardite would be X or Y. In the case that a sequel came where both can be caught at one time. (Assuming you could also get 2 masterballs to catch them) Their energies would merge the 2 halves together. In Y, the the X stone does work. In Z, you need yveltal and xerneas in your party as well as mewtwo holding mewtwonite x or y (if x the y stone has to be in your inventory and vice versa) and you have to mega evolve mewtwo in battle, this will make a dialogue like. "Energy is radiating off Xerneas and Yveltal! Mewtwonite X & Y are reacting to [PLAYERNAME]'s mega ring! Mewtwo mega evolved into mega mewtwo!" A similar result happens when you use this process on charizard. This mega mewtwo isn't x or y but a perfect version of mega mewtwo, mega charizard isn't x or y but a perfect version if it.
The only examples I could find were from the internet. I give credit to the original makers of the images.