Just installed Syncthing on my Scale server. It looks like it doesn’t have users but rather folder IDs that are then used to sync devices. One of the cool features of Nextcloud is the ability to share files with other users. Can this be done with Syncthing?
Where do you want to sync the phones to? I assume you have a server where you want to keep these, otherwise my_folder is just your phone’s local storage.
With that in mind what you described is a very straightforward synching configuration. You install it on the server, give it access to the three folders, install it on both phones, and configure it to sync:
Don’t understand why you think you need users for that.