I’m going to refer to this second “God” as a Demiurge, as that seems like the most appropriate word to use and to avoid confusion.
Assume that God creates this Demiurge, and the Demiurge then creates a parallel universe called Gaia. What is the First Cause of everything in Gaia? You might be tempted to attribute this to the Demiurge, but the Demiurge is himself not uncaused, correct? The Demiurge’s existence is thus caused by God, such that God remains the First Cause of all things in Gaia and, indeed, anywhere, whatever parallel realities there are. The Demiurge is still not the First Cause of all things that are and could possibly be. While immensely powerful, he is not omnipotent for that reason. The Demiurge’s causal power remains derivative from God causing his existence, and so the Demiurge remains a secondary or instrumental cause of the things he does.
I’ve referred to the First Cause argument, but if you are familiar with the argument from composite things or from contingency, those would apply to here as well. God is not simply a most powerful being among other beings. While He is most powerful, that’s not the reason we call Him God per se.