You don't need to see the dash cam. It could be in back of some other device.
Is there some way of having one monitor for both rear views that can be switched between them? It seems to me you'd rarely want both simultaneously, and on the rare cases where you might it would be in a situation where a couple of moments fiddling would be acceptable (i.e. close maneuvering where it's going to take a little time regardless).
Many aftermarket car radios, particularly double-DIN ones with DVD players etc., have provision for a rear camera input.
Dutchmen -- Surely part of being skilled is knowing what tools make you more effective and using them when appropriate?