tl;dr:
They are compatible, n34 follow the email workflow while ngit follow the branch-PR-merge workflow.
Yes, they are both compatible, we both follow NIP-34. Using both you can send and fetch patch from relays, the only difference is how the patches being sent and fetched.
Ngit will handle the git things for you, like you only have to make your changes in pr/* branch, then push it, its remote will make the patch events for you and broadcast it to the relays.
n34 not, it just handle the nostr part, you have to make the patch my yourself using git-format-patch, then provide the patches to n34 to make the events and broadcast it.
Same with fetching, ngit will fetch the patches and apply them to branches, but n34 will give you the patches file and you can do whatever you want with them, like applying them using git-am.