I have a similar setup and can view both cameras. Unfortunately I rarely use them so am not an expert.
Not sure if this helps....
Cam Viewer - Live View and PiP
With your Dash Cam successfully connected, you will be able to see the live image your Dash Cam is
seeing by using the Live View Screen on the Cam Viewer app.
With a Rear Camera attached, you will be able to see the live image from both the Front Facing Dash Cam and the Rear Camera, which will appear on your Live View as a PiP (Picture in Picture) in the top left.
With a Rear Cam attached, you can tap the Rear Camera PiP in the Live View Image to make the
Rear Camera fill the current viewing screen. Tapping again will reduce the Rear Camera view to
its original size and position.