The new AIRBUS A 400M heavy airlifter will have counter rotation dual props to cancel the torque generated by the 11,000 shaft horsepower per side turbines, also to cancel the prop tip vorticies - nothing new here - the Rusky'ss been doing this for years on the bear.. Concerning small twins - it is done to cancel low speed additive "P"-factor - IE t improve low speed directional stability - I think ..