Having been a police officer AND having supervised the department's safety section, I learned to absolutely forbid that practice. It works just fine as long as there's NEVER anything unexpected or an emergency. But when something unexpected happens, you may stomp down on the brake with your left foot AND the gas with your right foot at the same time. In an unexpected emergency, or even in a collision, there's a tendency to push down with both feet, and that's OK as long as neither foot is near the accelerator pedal.