Bargain

With the switching, if there are two inlets there should be some sort of switch to change from one to the other. It won't be split screen but instead display one or the other at a time.
If there is no switch, you may find that you have to remove the camera cable before it will display the other, my GPS is like this, which is pointless.
I like the idea about mounting one on the front for 4 wlding, but it would need to be pretty water tight including you connections. This would go for the rear one as well, I assume they only use RCA cables?
If the camera is water tight you could always look at upgrading you connections, it would still work out to be a good value system.
I wonder if you could heat shrink the connections?