Other Entireties

An Entirety consists of all universes, all timelines, all dimensions, all Realities.

...So what happens when an author arbitrarily decides to say "okay but here's a second one?"

An Entirety's boundaries are defined exclusively by the fact that one cannot travel between them under any circumstances. Except for like extremely specific circumstances like being an agent of Chaos who is not bound to any Entirety. But OTHER THAN THAT, no crossing whatsoever!! If someone ever pulled that off you would now just have one Entirety, not two. Boy, I bet that would make for a pretty great plot twist, to spend literal years pretending crossing Entireties was real only to reveal that that would cause a merge instead and now Some Shit is about to happen!

Haha yeah that would be wild well I'm sure something like that would never happ