How would you know that merging code from the first bugfix wouldn't break the (just tested) code from the second bugfix?? Would you assume that the first bugfix will be merged first and branch off of that?
Branch from master, and rerun tests before the final merge, like you should in any other software? (Many processes fail that criterion, see https://bors.tech/ for something that gets this right).
Ideally you work on a different enough bug that there's limited interaction, and ideally that's figured out before you fix it, but those criteria are indeed harder to satisfy in a bigger software.