Multiple cams on a single computer isn't an official feature and hasn't really been thoroughly tested, but several users have reported success getting it to work on both Windows and Mac. I'll do my best to help you get it working. If anybody else has experience getting multiple cams working, please chime in
On the windows version, you should be able to run iCamSource, and then, once the first iCamSource window is up, run iCamSource again. Then, there should be 2 iCamSource windows showing.
It's a little different on the Mac version. You can't just run the same executable twice like you can on Windows. On Mac, you need to make a copy of iCamSource.app, then run both the original and the copy in order to have 2 iCamSource windows running.
Once there are 2 iCamSource windows showing, you should set one window to use camera #1 and the other window to use camera #2. If you want to view both of them from the same iPhone, make sure to set the login/password to be exactly the same on both iCamSource windows.
If it still doesn't work for you after following those steps, then I have some questions for you:
Are you using the Mac or Windows version?
What error message are seeing exactly?