It's between the realities - As characters grow closer, their realities grow closer as well.

Mergers are rather unique to the pair who are merging, because it's worked out between the muns. Sometimes it's something subtle like a different language on signs, or certain flora growing somewhere it shouldn't. Sometimes it's more drastic, such as buildings being replaced by a building from the other reality. The amount that is merged depends on the strength of the relationship.


This post might help to give you a better idea of what can change and why.