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Dual Motor Efficiency?

Everyone seems to expect the dual motor Model 3 to be more efficient than the RWD just like the Model S (5% efficiency gain). However the Model 3 already uses a more efficient permanent magnet motor so it seems like there might not be as much to gain by having a second motor with different gearing. Do permanent magnet motors have more loss at zero load (torque sleep?) than the inductive motors? Maybe they could gain efficiency by having a smaller permanent magnet motor and a small inductive motor. Thoughts?
Twice as many motors means halfish the current to each. Half the current is 1/4 the resistive loss × two motors = 1/2 resistive loss overall. Some added loss due to drive unit friction. So AWD should be more efficient, but exact gain depends on the motor efficency curve.

PM motor with no drive signal will have very low drag, but possibly higher than undriven AC inductive due to (small) eddy currents from the magnets.
PM is more efficient, easier to cool, and simpler to drive than AC induction, so no gain in using AC induction for front motor.