We're working on cultural unity as well. Single religion is pretty much dominant in both realms (and will be in the third as well) at the moment, and I'm working on trying to build that up so it actually has a real culture behind it (how villages/cities are organized, what exactly the temples and shrines look like, etc). Beyond that, we have some interrealm communication going on, and working on Shinnen together should help that.
The most important thing would be making sure that people in power keep teaching their underlings how things should be, in order that everyone is well aware of the structure and the uses of the realm. It is pretty easy in bm for two realms that are very closely tied (perhaps a colony and motherland) to separate in time, just because no one takes care to explain what is their history constantly.
