I might just not be following correctly but committing in git just carries the branch along for the ride, so there's nothing to do in git for this scenario.
IIRC forcing some specific branch name to point to my changes with `jj` was non-obvious and what made me give up and go back to git when I tried it last year.
>>wild_e+(OP)
You are mistaken. In the workflow I described, I am making changes on top of all branches at once and then deciding which branch to send the new commit to. This allows me to make changes simultaneously to both branches without friction.