Driving autos with both feet is a definite advantage. I drove professionally for a while, in an auto, and taught myself to left-foot brake almost immediately. It provides big advantages in smoothness and reaction time.
The reason that driving instructors don't teach it is that it is a specialist skill that most drivers simply can't cope with. I've come to the conclusion recently that most people simply can't drive. They are unable to cope with the multiplicity of stimuli/responses that they have to process continually, and so they just don't. This is why people will pull out in front of you on motorways, or fail to stop when the traffic lights turn red. It's why people voluntarily go out and buy Vauxhall Corsas.