HI Paddy
The simple answer is yes
I have just fitted a twin lense camera to a mirror monitor
One camera used full time looking back and one comes on when reverse selected. There is a override button on the monitor should you wish to switch tha camera manually
I bought mine from
http://www.2seetv.co.uk/
Not that cheap if you compare to stuff on e-Bay but I think that it is much better quality and the after sales service was very good and helpful
David