Let’s say you have two computers, and a folder in a usb stick. You set it as a syncthing folder in one computer. Can you then, set the same usb folder as a syncthing folder, in the second computer? Should this work fine?
Your question is not clear, syncthing syncs files from one computer to another, where the directory is on the disk drive or a usb, it doesn’t really matter.
The question is very clear. In other words, will there be an issue, if the same folder is being synced by two different systems? i.e., if that folder is located in a removable device, which can be inserted in more than one syncthing system.
Perhaps the hidden syncthing configuration files which are placed in the folder, are specific to one installation, in which case, if a different syncthing system tries to use that folder and parses those config files, there might be an issue?
This will work, however Syncthing will complain that the folder path is missing when you swap the USB drive to another device so you may have some manual involvement to clear the error. Syncthing won’t know that it’s being used on different device as the index files are on the host device.
All Syncthing cares about is the hidden folder on the target location called .stfolder in order to function.
Depending on your needs, I think Synctoy may be more suitable as that expects devices to be unplugged and will sync automatically when the USB drive reappears. But needs to be in a Windows environment.