One simulator would just "run the laws of our universe" (I don't care to make that precise, since presently it's a trivial statement, but hopefully it's clear that I mean to distinguish it from running a computer), but then that sort of trivializes the idea of "simulation".

Maybe it's possible for us to create a sub-universe of ours with a quantum computer such that we view the entities as a part of our universe, but the entities cannot be aware of us. (This is the insects at the surface of a pond idea, unaware of the dimension above or below.)