If you have two gens/govs in the same stack or city, you click on one, find the ancilary you want to give to the other guy, click on that ancilary, drag it down to the bottom where all your units (including the other gen/gov) are, and drop the ancilary on the other gen.

It's just dragging and dropping, just with people now.

I'm very leary about choosing heirs. My experience has been that every time I choose who should be the next leader, that person has ZERO chiildren. Even when I find one who has the Fertlile v/v, they still don't have any kids. Like literally, I've choose an heir who was 18, had the Fertile v/v. He was married by the time he was 20, to a girl of similar age, and for fourty years they didn't have one kid... I really hate how few of the integral game mechanics are explained in the manual or in the too-rare dev posts.