With the multiverse all things are possible. The "mirror" universe could just be whatever multiverse happens to be the antipode of the current zeitgeist. Like a sociological dipole. Therefore appearing to be "recreated" as a psycho-re-active kind of place, but also a physical reality that always existed. This could be happening often enough through various combinations that the "mirror" universe would even have a memory of previous encounters with a "prime" universe similar to, but not necessarily identical to, the prime universe we are currently watching.