How do I display both the front and reverse cameras on the display screen at the same time?

I have a Rexing V3 full featured model with the GPS logger. This is the model that has two lenses: a front view and a cabin view lens. I know the display can be configured to show a split screen, showing the view of both cameras.

I have configured it the way I wanted it. Everything is working fine but there is one thing I cannot figure out:  

How do I set up the screen to show both cameras? 

thanks.