Why is it you drive with only one foot instead of both?

You are not supposed to brake with your other foot. You could mistakenly press the accelerator while trying to stop. By using only one foot, you have to remove your foot from the "gas pedal" in order to brake.

The reason you drive with one foot is because before there were automatic transmission cars, you needed to depress the clutch to shift gears in the standard transmission vehicles. The main foot is used alternately between the accelerator and the brake. This link even has video of the heel-toe maneuvur.
http://www.standardshift.com/videos.html

You can tell when the person in front is driving with two feet in an automatic by the amount of braking going on without the car actually slowing down. This causes unnecessary wear on the brakes, and can give a 'crying wolf' message to the driver behind. A good reason for sticking to the one foot for brake and accelerator is that you will have less trouble when in an emergency (or on a holiday...) you have to drive a car with a clutch. Source: over 1,000,000 miles on the road in both automatics and manuals (and occasionally those horrible things with a gear change and no clutch - yuck!), and even a tractor with a clutch lever.

http://ask.metafilter.com/72034/Gimme-brakes