Exactly that, i was playing as harold and i let william win, he olny won the title of king of england and the barony of westminster, i kept almost the 50% of the lands of england. The thing is that i have galvekind and all my lands will be splitted up between my sons, i know how to change the law in my primary title (duke of oxford) but im not shure if this will change the succesion in all my others duchies.
I'm fairly sure the example was used of having different duchies descending by different methods, and thus although you could gather them together in one reign, they'd risk splitting up in the next unless you could align the succession methods.