I have the Pep Boys $99 special that is wireless on my truck, mounted in the rear liscence plate mount and it works very well for backing up. It has the disadvantage of picking up some wireless security cameras as I drive along. Several of the guys here on the forum have them for their cars. They work well for backing up. You know just how far to go once used, and yes I use my side mirrors.
Now for driving, that is another matter. I ruined a Lotus turbo Esprit with the dinky side mirrors they provide. Before sunup one morning I wanted to change lanes. I couldn't tell where the lights in the lane next to me were, and when I looked forward, the truck in front of me had stopped. Had no room or time to maneuver. Don't know that a cam would have helped in the dark. The depth perception is a problem, but it tells you if someone is on your butt or coming up on your side. I don't think depth perception matters when someone is closing the distance on your side, you know it and should know not to do it. You have to make the decision whether you have enough room to change lanes or not. That is what blinkers are for, to warn the other driver(s) what your intention is. I don't think two cameras would help at all. It just gives you two pictures that don't have depth perception, and at its worst would be a distraction to everything else around you. It is like any other device we use. It just takes some getting used to it, and should be used as an aid rather than a dependence. I will use the side mirrors, it is just that I don't care for them on the car. I think they look better without the mirrors. That is why I like the ones shown on this thread because they are easily removed.
Now, where would I put them? I have seen several places on the car. If you want it to aid your side mirrors, it needs to be higher than the bottom of the rear clip. I wouldn't trust my own eyes at that level while moving forward. Where I considering is on top of the spider just at the break of the rear clip, or on the top of the rear clip just above the plexi cover. I would mold it into the body work to give it a good look. I wouldn't put it under the rear plexi as there is too much distortion and/or degredation of the picture. I even thought of using a third brake light and mounting the camera inside it(assuming it would fit). It would be glassed into the spider to give it a smooth aperance. When I send the car to the painters for some touchup work and doing some flare work, I'll have decided where the best place will be.
Bill