But how do you prevent the conquerers from considering themselves Chinese (OTL) and then proceeding to unify China?
Presumably, you'd need a conquering people with enough strength to take the north of OTL China - and to hold it for a considerable time - but lacking the capacity or the will to take the south as well. If China were to be divided in this way for long enough, the north might well orient (...septentrionate?) itself towards the cultural region of the conquerors, while the south (just as unable to retake the north as the north is to take the south) instead casts its gaze further southward. There
were (some say
are, but I can't speak to that) of course cultural differences bewtween north and south. The north would presumably be more integrated in the horse culture of the northern steppe peoples, and have predominantly wheat-based agriculture. The south is territorially unfit for that steppe horse-culture, and might realistically have a more naval attitude than the north is likely to develop. It's agriculture would be more rice-based. If the separation lasts long enough, you'd end up with two very different countries.
Of course, both states might well end up considering themselves the
real China, with the rulers of both claiming the mandate of heaven.