According to Andrea (who is a scientist and possibly has a degree in this subject) every universe have at least two sides - two sides are already proven, there are theories that more sides exist but without any experimental proof.
It can be imagined as if the universe was a coin (or, in case of multiple sides, dice). Every side looks like separate universe (separate for most practical purposes, as Tara said) but are actually part of same reality and share a magic bond.
Magic actually flows from one side to the other in specific places. In case of Main Universe and it's other half, one of those places is (in) Moperville, or at least was - something happened which stopped the flow and was detectable as a magic energy clog from the other side and as a excess magic energy on Moperville's side.
It is also possible to travel between the halves of the universe using special places (possibly not identical to the places where magic flows through).