Its an unlikely event that both would be editing the same file at the same time. I think I've achieved this. Been messing with Syncthing today. we each have a "shared" folder on our phones and the server has one too. I selected to share with both devices.
The trick now is when you have a PC with multiple users that want to share that "shared" folder. I need to work that one out ... :-)
Mmm, Seafile is is developed by an for-profit organisation. Looks interesting but might stick with nextcloud if I have to move to Seafile. Syncthing seems really robust and simple. I think its just the file sharing bit that I'm missing. Nextcloud is just a beast.