I started years ago in the winter, automatic trans, choke not working right. I needed to give the car gas to keep it running even when stopped, therefore needed both feet. I do not rest my left foot on the brake, I guess years of using it built up the leg so that I keep my foot about a quarter of an inch above the brake pedal. I still do it since I believe that I can hit the brakes much faster. If you use only your right foot you need to lift off the gas, move it to the left, then press down. Using my left foot for braking all I need to do is press down, eliminates the lift up off the gas, and over.
For manual transmission cars I use my right foot to brake.
Because I have driven both manual and automatics, in a pure panic situation I end up with my left foot on the brake, and my right foot on top of my left foot, left foot always gets there first.
I do not know why they teach people to use only the right foot. You always hear of people saying that the car accelerated by itself, but I expect that in 99% of the cases, the person hit the gas instead of the brake with their right foot. Why is it that the brake pedal is usually higher that the gas? It appears to me that in many cars if you step on the brake and your foot slips off the pedal you may slip right onto the gas and floor it.....