just wondering if you can actually trade the stones across and get mega mewtwo x on y, and vice-versa
Yes you can, because they are hold able items.
Hope this helps!
Yes, you can. You also don't have to give a Mega Stone to that specific Pokémon to trade it. For example, you can give your Charizardnite X to one of the excessive Bunnelby's you'll catch, and trade THAT over to Pokémon Y.