By default we detect webcams by their ID. If you have multiple identical webcams, this can lead to the problem that only one of them is visible. In this case, you have to change the detection to the connected port.
To switch to path based detection, log in via ssh console and edit the webcam configuration:
sudo nano /usr/local/Repetier-Setup/etc/webcam.conf
Near the end you have to change WEBCAM_DIR to:
Press control + x to save changes and exit.
sudo rm -r /usr/local/Repetier-Setup/etc/webcam_ports cd /usr/local/Repetier-Setup/bin sudo ./startAllCams
Then you can reload your web config in your browser and you should now see the new list of webcam names. From there on you must always use the same ports for your webcams.