bar is a descendant of foo, correct? That's the situation I'm talking about.

Assuming that's the case, you want to make #2 a local PR anyway -- that is, its source is you/bar and its target is you/foo. Once #1 gets merged to them/main (by someone with the access to the upstream repo, "them"), #2 will (I predict, based on reasons I've already given) automatically become a PR "in" the repo them. That is, its target branch will automatically change to them/main.