Cultures in our world collapse and rebuild and change, and during civil wars, even more so successful ones, divide cultures. When they split, I think the fair thing is that both keep 2/3rds or 3/4ths of the cultural technologies since this could add an interpretations that the exclusive one were made mostly by that part of the culture and the common ones were widely known throughout.

But I would lie if I said I want this because of realism. My playstyle (and for sure that of many others) is hurt by the way cultures currently work.

I put a few humans into a single village and watch how they spread over the world. With no way of culture dividing or collapsing, they are monolithic from start to finish.