Many worlds ontological argument
The many worlds ontological argument is based on the possibility of God's existence in some worlds implies his existence in all possible worlds.
"We have here the germ of the ontological argument for God's existence. For if it is possible that God exists, there is a possible world in which God has necessary existence. But then He exists in every world, including this one. Thus, the atheist is thrust into the awkward position of having to say that God's existence is impossible."
Entities in possible but non-existent worlds have no bearing on the actual world. The argument needs to establish that all or most other possible worlds exist.
This is a proof by logic that attempts to define God into existence.
Arguably, the concept of God put forward in the argument is incoherent and self-contradictory. This probably would make it impossible for this God to exist in any possible world. Other conceptions of God may be possible, just not this one.
Even if God had a necessary existence in a possible world, it does not follow that could would exist in all other worlds.
There may be worlds in which God is necessarily non-existent, which rules out his existence in all possible universes by the same logic.