In the words of Elon"Because we have two drive units, where we can shift the power from front to rear, and constantly be at the optimal efficiency point for each motor. We're actually able to overcome the penalty of the increased mass of the motor. We're able to balance the efficiency of the motors....If you have just one motor, it's always on a particular power-vs-efficiency curve.But if you have two motors, you can optimize between them, and have the motors operate in their more efficient regime more of the time"