Just for clarity.
The NWS and rudder don't actually sum. They remain independent but can work against each other. The two tillers do sum each other and there is no indication of dual input.
Also remember that the rudder is ineffective at low speed and the tiller should not be used at high speed.
Don't forget you also have differential braking for directional control. I don't believe the toe brakes "sum" (Maybe wrong) but if one pilot presses left brake and the other presses right brake the aircraft will stop in a straight line if the pressures are equal.
Sounds complicated but I've never heard of anybody having issues with directional control.