How would two different but similar states fuse together ?

Hi guys, I'm working on a Pokemon worldbuilding, and it involves two regions having been one for centuries, but that have gone different paths and end up being very different from one another at the moment they want to merge together back.

To give you an exemple, it would be like having North and South Korea saying that they want to be one state again, but both people and governement willing to keep their way of living and organization, while admitting that the other's is at least valid, if not respectable.

Could such a thing work ? What would be the system to keep it together in such cases ?
 
I would assume federally is the way most likely way for reunification to happen in this type of situation if its a union of equals.
 
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