Are both cameras plugged directly into the computer, or into a USB hub? If they are plugged into a USB hub, try plugging them directly into the computer.
Is it a desktop Mac, Mac mini, or MacBook? Do you have any other software running that might be attempting to use the webcam? What version of OS X are you running? Have you installed all of the latest Mac OS X updates? Do you have any other webcams available to test with?