My camera system has the abillity to handle four cameras and can run a split screen on the mirror. They have very small cameras that can be stuck to a side body panel and look backwards, but unless you have a black car that wouldn't look too great. It would be interesting to remove the side mirrors entirely and replace them with a cool small aerodynamic bracket that incorporates a camera.
I would not use the Master Tailgaters mirror system that I bought, the mirror images are poor. In time I'm going to replace it with different system because the rear view improves dramatically.