We’ve heard reports of problems of certain Mac models that prevent multiple USB cameras working simultaneously due to an issue with the USB bus. It isn’t a problem with iCamSource in particular, because the problem occurs with other programs as well.
You can find out if this is the problem you are experiencing by trying to open two of the cameras in different programs simultaneously (such as FaceTime or Photobooth). For example, open one camera in Photobooth and then try to open a different camera in FaceTime.