[MetalMan]

The reason why it can't be done all the time is because the controllers need to sense the position of the rotor. If there are two motors, and they aren't spinning at the same speed, then the controller can't sense the position of the rotors. But in an E-Maxx, the motors have to spin at the same speed (because of the gearing), and so the controller can sense the rotors' position more easily/properly.