The multiverse spacetime paradox?
Read Edwin Abbot's book Flatland.

In this story we visit a vision of a two dimensional "Universe" embedded in a higher dimensional space. Now by analogy we may suppose that our Universe is embedded in a higher dimension space with plenty of room for other (3+1)-Universes.

I do not claim this is a viable model of "reality" just a thought experiment to show "where" other universes might be if there are indeed multiple universes. Also Flatland is a fun read.