Using both feet is a bad habit to pick up. Driving with Left on break and right on gas means there is the possibility of both of them at the same time (not to mention you still need to swerve out of the way if this is an option).
The brain is not very good at multitasking reactions, and the simple action of pulling up the right and pressing the left is quite complicated, and would take many hours of practicing this action (which is not very good for the car or the tires, and quite a use of gas due to constant acceleration)
In an emergency situation where you need to stop, you perform 2 actions,
1. stopping your acceleration by releasing the gas pedal
2. stopping you forward momentum by pressing the break
unless you perform these actions with flawless precision, you will hit the break while still pressing the gas, and that split second could mean a few extra feet and the possibility of killing someone. Even a 5mph difference in speed could mean a full recovery or paralyzation for life.