It's not going to make the car blow up or anything but it's not the safest way to drive. A standard car has a clutch which is designed ot be used by the left foot, often people that are used to driving a standard car have trouble not using their left foot when they switch back to an automatic car. If it works than there really isn't any danger in it, the concern is becoming confused when attempting to avoid a collision. If you panic and your feet are mixed up you may not be able to successfully avoid the accident. Another potential issue is if you accelerate while stepping on the brake, this can cause damage to a number of areas in the car.
You should learn to use only one foot but it is not a terrible thing to use two. It's still better than none although I suppose using no feet would actually be the safest way to drive.
