Normally, you would add a folder on one side, then share it with a remote device, and then on that device accept it under a pre-existing path. If you’ve already added them to Syncthing independently though, you can still “link” them by using the same folder ID. The folder ID can be edited in the Advanced Configuration, although I would suggest just removing the current ones, and then re-add them using the first method, especially if you’re not really familiar with Syncthing yet.