From repo maintenance point of view it is better to have smaller commits and pull request (PR) focused on resolving a specific issue or new feature. Large PR may have small problematic parts that may prevent accepting a merge. So 7 and 8 PR should be submitted separately. Practically, from committer point of view, it is important to create feature branches, you can keep working on the next thing when previous feature branch is waiting for PR merge.
Other consideration Mike did not mention is if we should primarily work on `master` SFX8 branch, then backport what is common to SFX2. That would possibly simplify SFX8 development but complicate significantly support for SFX2.
I think the approach that Mike is proposing keeps both branches alive and we should use it since we still actively support both 7 & 8.
Jarek