You can now evolve your Mewtwo into Mega Mewtwo X or Mega Mewtwo Y in Pokémon Sun and Moon!
A Mystery gift code has been released granting both Mewtwonite X or Mewtwonite Y so you can choose which evolution you wish your Mewtwo to have during battle. Mewtwo is not able to be obtained in Alola (yet) but this will work on your Mewtwo you have ported over using Poké Transporter and Pokémon Bank from an earlier game.
Any items your Pokémon carry are lost if you transfer them between games, so this is currently the only way to obtain mega evolution power for your Mewtwo in the current titles.
The Mystery Gift code you need is M2DESCENT