As has been discussed on other threads, the motivation to use contra-rotating prop (coaxial shafts) is to INCREASE PROPULSION EFFICIENCY. A conventional prop creates a rotating slipstream vortex, which represents a loss of useful energy. Adding the second prop assembly gives the opportunity to straighten out this vortex and recover that lost energy. (In a turbofan engine, the fan stator vanes serve the same function.)
And, in a single-engine type, it's that rotating vortex which acts against the vertical fin & rudder that requires the pilot apply a rudder input to keep the airplane straight on takeoff. It's called "torque", but that label is obfuscation. It's the prop vortex, and a contrarotating prop overcomes the situation.
