First check with ls -l /dev/video* if both appear. Are you using the latest pi image which has new webcam handling. It uses webcams listen in /dev/v4l/by-id/ so check with
ls /dev/v4l/by-id/
You can also change it to use webcam by path whcih might be required if id is identical:
ls /dev/v4l/by-path/
Whcih one is used is defined in /usr/local/Repetier-Setup/etc/webcam.conf