>>37004068You could do this with Hacking in XY-USUM where you can Mega Evolve a Pokemon in its Megs Evolution stat.
Just like how Imposter Chansey in Hackmons and Balanced Hackmons still gets the Eviolite boost, Mega Evolution still thinks Mewtwo is in its regular form.
Mega Evolving a Mega only really changes the abilities for most in Hackmons, and here it is only useful on Mewtwo and Charizard since you can’t bring Illusion or Drizzle then Mega Evolve.