Not quite following you there, let me see if my system helps.
I have a NAS with multiple syncthing instances (services) running:
- me, my laptops, my phone
- my partner, their laptop & phone
- other equipment in the house (for Music, etc) For me & my partner, I added a shared folder that both syncthing instances used.
It works fine if you ignore file ownerships and set the underlying permissions so both users (ie the group users) have write access.
But, if we're both editing something, syncthing will nuke one of them (so I have versioning on). But for us this never happens as they're rarely writing to those files.