Acceleration (gas) and brakes are two opposite process while
operating a vehicle. Acceleration is done to increase speed and brake to
decrease the speed of a vehicle. You either need to increase or decrease the speed
of your vehicle at a time.

When you press gas, you are burning fuel and increasing your
speed. As you apply brakes, the momentum you’ve gained by the fuel is
decreased. Hence the lesser you apply brakes, the lesser you’ll spend on fuel.

When you use both feet for gas and brakes, you will be
pressing both feet at the same time unknowingly (even a small dab on the pedal
will activate it), which will

1. Deteriorate your transmission.
2. Put the brake lights on, even when you are accelerating, putting the vehicle
   behind you and yourself at risk.
3. Make you spend more money on gas.
4. They are opposites. They don’t go together.
5. Put you to high risk, when you need to apply sudden brakes. You tend to press
   both your feet.
6. Wear out your brakes, eventually leading to break fluid leakage and break fails.
   

Using two feet is never recommended. Even in manual transmission, you use ONLY ONE foot for Brake and accelerator (gas). You use other foot for clutch during gear shifts.

If you’ve learned driving yourself or your instructor didn’t pay much attention to how many feet you were using, you may have gotten yourself into a dangerous practice; even though you’ve mastered driving using two feet for brake and accelerator.

My two cents.