Thanks for all the advice. In the end it split. I tried to pass Late feudal but the council voted against - I wasn't very popular and couldn't buy enough votes
The Bishop thing sounds interesting - I might try that at some point. Gavelkind is hard to keep conquests together, so the Empire route sounds a good way to go. Unfortunately for me, consumption took my ruler before he could finish off his empire plan!
just war with claims on the other kingdom, gavelkind isn't that bad if you're willing to be at war with your family/religion, etc.
