Would this work (in theory)???

Question

So if a Ditto was holding a Gardevoirite (or any mega stone) and then transformed into a Gardevoir (or any other Pokemon that can mega evolve), would it be able to mega evolve?

Answer 1

Simply, No.
>A Pokemon that can mega evolve must be holding it's respective stone to mega evolve, and transformed Pokemon can NOT mega evolve even if they are holding the stone.

But this scenario can take place,
>a Pokemon can transform into an already mega evolved Pokemon by transforming into the opposing Pokemon but some abilities can not be copied. eg: Parental Bond can not be copied so a ditto that has mega evolved to mega kanga can not hit twice.

Hope I helped:)
Source:Experience

Comments

Would that count as your teams mega evolution? I'm wondering if using this method I could have two mega Pokemon at once.

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There can be up to 6 mega evolved Pokemon on a field at once.  Triple battle, 4-5 dittos, 1-2 megas.  Of course this doesn't work ingame and in any competitive group except ordinary link battles with friends.  The battle would need to be organized and produced so that both sides would cooperate.  

Ingame it is possible to fight almost all double/triple battle trainers with 2-3 megas if ditto is used.

Answer 2

No. It won't be able to mega evolve even if it is holding a mega stone and turns into that Pokemon.
Source:Just tested it on showdown.
Hope I helped.