Megas with Z-Moves

[Astraleevee]

Hey everybody, I haven’t played in a while so this could be a stupid question, I know that you cannot give a Pokémon both a mega stone & and a Z-Crystal. However, in a double battle could you have say Blastoise mega evolve, as well as a Pokémon such as Blissey with the Waterium Z. Then have Blissey use bestow to give Mega Blastoise the Waterium Z thus allowing it to use Hydro Vortex.
I myself cannot not test it since I have not got a game since Alpha Sapphire, I was just curious

 

[Astraleevee]

P.S. sorry if I posted in the wrong spot

 

[Sceptile Leaf Blade]

Mega pokémon can't use Z-Moves no matter what. Mega Rayquaza can't do it. Ditto transforming into a mega evolved pokémon while holding a Z-crystal can't do it. Also, you can't move Z-crystals in battle from pokémon to pokémon using moves or techniques like Trick, Bestow, Switcheroo, Symbiosis, etc. because it's not really the pokémon that's holding the Z-crystal, the trainer holds the Z-crystal in the Z-ring, it just takes up the held item slot for balance reasons. Same for Primal Reversions, they can't use Z-Moves either. Waterium Z Ditto transforming into Primal Kyogre still can't use Hydro Vortex for instance.

 

[OshyHikari]

Megas/Primals already have to hold an item to access their Mega/Primal form, so they can't even use the Z-Crystal to begin with. Game Freak were even smart enough to make Rayquaza unable to Mega Evolve if it held a Z-Crystal, making the Z-Move take priority over the Mega.

It's clear that a Mega that was able to use a Z-Move would be incredibly broken, which likely is what caused that example with Rayquaza.