Im having a hard time understanding the constellation of your current setup. From what i understand you have:
- You and your wifes phones
- A shared laptop with two accounts
- A server running FBQ and or syncthing?
The way i see it you could just use syncthing for all of these, no need for FBQ.
If you need FBQ though i dont think having a single folder synced by both FBQ and syncthing will cause any issues as long as they arent acting in parallel.
So device1 <syncthing> device2 <FBQ> device3 should be fine,
but device1 <syncthing> device2 <FBQ + syncthing> device3 might cause issues in my experience with doing the same with nextcloud.
I have a setup where i use a nextcloud server that keeps file version history. Then i have a laptop that is connected to the server through nextcloud. That laptop then syncs that same nextcloud connected folder through syncthing to like ~10 other laptops. So basically all the laptops talk to each other to stay in sync through syncthing and then one of those laptops also uses the nextcloud app to sync this syncthing cluster to a nextcloud server for version control.