But I think that’s not the best way to get shadow mewtwo two it be cooler if the player finds shadow mewtwo two where he found mew and finds it where he found mewtwo and then it attacks be Couse it was looking for mew and mewtwo and it attackt because it knows you have them (it will be at the mew spot after the rematches and then you have to have mew and mewtwo in party so it will be where mewtwo was found then it will attack and you can catch it )( the quests name the shadow from space)
Victini is where you find mew after the rematches.
Make it after that
Or maybe they could make it that If you take Mewtwo to Cidadark Isle in Void Gauntlet,an event will occur that transforms Mewtwo into Shadow Mewtwo.