You see, in gen 7 Ubers I need Ultra Necrozma in place of Necrozma-Dusk Mane and I'm using Normalium Z Arceus. I have some awesome calculations in a way I have 81% chance to OHKO defensive Zygarde with Z Extreme Speed (I'll mention how if anyone is interested).
So should I go forward with it?
You can also mention how would it be other tiers. Thanks in advance!
You really should not be using 2 Z-moves in almost any situation. I think that Arceus using Ice Beam (2HKOes Zygarde-C with a Life Orb attached) and Leftovers/Chople Berry/Life Orb is not as bad as having to choose between activating the Z-move on your Arceus or Necrozma-Dusk Mane.
You can still do it, but you will only be able to use one of the two z-moves. I tried 6 electrium z's on my Arceus team, and I could still use it! It's viable, but it might not be the best choice, to be honest... Still, you could give itg a go. Good luck!
PS: Hope this helped!