Do you have any other USB devices plugged into the iMac at the same time? If so, you might try unplugging them.
A small number of users have experienced this kind of issue, and we believe it has something to do with the power required by the multiple cameras along the USB bus. What is the make and model of the USB camera? Do the two webcams work simultaneously with any other applications, say running Photo Booth and iChat at the same time, choosing a separate camera in each?