>>20243399Unless you're trying to turn one of the Zards or Mewtwos into the opposite Mega, I'd assume the standard Mega Evolution sequence happens and the Pokemon "Mega Evolves" into the form you already forced it to use. After all, a Mega Gardevoir is still considered a Gardevoir; why shouldn't it Mega Evolve like any other Gardevoir holding a Gardevoirite would?
MCY <--> MCX and MMY <--> MMX probably trades one forme for the other. Outside of going Yzard -> Xzard for serious Sun-boosted STABs, there's not much of a point to using a Mega stone instead of, say, a Life Orb or Leftovers or something.
Of course, I could be completely wrong about this.
