Many users have had success viewing multiple cameras on the same computer by running multiple instances of iCamSource.
To do that on Windows, simply run more than one instance of the iCamSource, enter the same login and password, and set up a different camera on each instance of iCamSource.
To do that on a Mac, you'll need to make a copy of the iCamSource application for each camera. First, run the first iCamSource and set up the first camera. Then run the second copy of iCamSource and set up the second camera.
In order to watch all of the cameras on your iPhone at the same time, make sure that all instances of iCamSource have the exact same login and password entered.