Repetier-Server allows you to use multiple webcams per printer. But if your webcam only supports YUYV mode and not MJPG it will send all data uncompressed so it puts a high load on your usb port. As a result you can only handle one webcam per USB port. So be sure that the webcams support MJPG mode.
To add an additional webcam for your printer go to Printer Settings Webcam and click Add Webcam:
You can add the new webcam just like the first one.
If you have multiple webcams set up, you can select them under Webcam:
To switch the webcam in Control, just click on the video: