If you have both cameras connected, but you only run one iCamSource, does the "No Audio" entry show up?
What happens if you plug in one camera, run iCamSource and select "No Audio" (and Start), then plug in the second camera and run iCamSource? Does "No Audio" show up in the list on the second iCamSource?
What are the entries that you see in the Audio drop-down list and the Video drop-down list? (other than "No Audio")
When both cameras are connected to the computer, are yo able to view them in other programs simultaneously? For example, can you open one camera in iChat and the other camera in Photobooth and have them work properly?
Do you have another Mac or another webcam available to test with?