My understanding of what's going on is still a bit sketchy
The cameras render in the order they were created with Cam2 set so that it does not clear the backbuffer (otherwise you'd only get what Cam2 can see).
Cam1 can see the blue cube only; Cam2 can see the red sphere only. You will note that the generated scene shows the blue cube with the red sphere overlayed: If you think of the red sphere as your GUI elements and Cam2 as your "GUIcam", you would simply squirrel these GUI-objects far away from the main in-game scene and camera (parenting both to Cam1 might be a good idea if Cam1 is able to travel far) so they get rendered in isolation.