Originally Posted by no4
... It would be a small problem to introduce a clutch or neutral gear to allow the prop to windmill freely and improve single engine performance.
A windmilling propeller has far more drag than a stopped (or feathered) propeller. That's why the first thing you do in an engine out event is stop the prop. A windmilling propeller has about as much drag as would a disc of the propeller's diameter. A stopped propeller has as much drag as the two (or three) blades do individually. You do the math - improving the single engine performance is the last thing that allowing the propeller to windmill would achieve.