Steve Foley wrote:

How about an engine with the crankshaft running out the front and back, so
you could mount a prop on the either end? That way you could have
counter-rotating props using the same engine.

Two props mounted on a common crank would turn in the same direction,
so they wouldn't be contra-rotating (I assume that is what you meant)
but might be counter-rotating if they turn in the opposite direction of
the other engine's props.

Come to think of it, if an engine had contra-props mounted front & back
(like the old Dornier Do X flying boat from the 30's) the props' torque
would cancel each other's out but would there still be a torque
reaction from the direction of engine rotation? I'm assuming the Do X
props were contra-rotating, not 100% sure on that.