My equipment is an iMac running OS 10.6.8. I've got iSight cam built-in and also connected a miscellaneous USB cam I had lying around (a cheap piece of junk I bought at an office supply store in Shanghai a few years ago...supposedly not Mac compatible). I also installed an application called Macam that is supposed to supply the drivers for various USB cams so they will work on macs.
Problem: I could not run my USB cam and the built-in iSight cam at the same time on iCamSource. Tried many different times, methods, orders of setting up, restarted the computer, shook my head vigorously, plugged into different USB outlets, etc., but as the other poster said, no matter what, only one cam would work at a time.
Weird solution: I read somewhere that running two instances of iCamSource at the same time on the same computer would be a possible workaround for this problem. I tried this out. Went to the application folder and duplicated the iCamSource application. Renamed it "iCamSource 2" (yes, I am a branding genius). I then opened both iCamSource and iCamSource 2. I then set up the iSight cam on iCamSource, and the USB cam on iCamSource 2. Worked perfectly. Both cameras could now be seen at the same time on iCam on my iphone and on my iPad.
What happened next?: For some reason, I then decided to give the correct method one more try, i.e. i started up on single instance of the iCamSource application. I added the iSight cam. Worked fine. I then added the USB cam to this same application. It also worked fine. Low and behold, both cams would now run simultaneously on one instance of iCamSource!
So, did I shake something loose with my weird solution? Accidentally install some dual USB driver thingy? Did my house get struck by lightning at that exact moment? Or is this just random luck? Who knows?! It works perfectly now!