Nose wheel steering?'s - A320
 

While taxiing using the tiller, what happens if you simultaneously use the rudder pedals? Does one overcome the other? Are they additive?

Lastly, (thankfully I have not ops tested this) but when accomplishing an auto land if one does not disconnect the autopilot while clearing the runway using the tiller, I have heard you can start to move the aircraft off centerline, but then it will quite violently steer back to centerline. Why is there a delay?

I think the tiller and NWS will sum - unless you have pressed the disconnect button on the NWS which will disconnect the rudder input so that you can do the Flt Ctl rudder check on the move.

It used to be that, on an auto land, if you did not disconnect the AP before you started to vacate the RW, you could override ROLLOUT and steer off the RW using NWS. Unfortunately, when you released the tiller, ROLLOUT would attempt to regain the CL using the NWS.
At some stage, Airbus redesigned the software so that, when 25 degrees off the RW CL, the AP would disconnect automatically. Happiness!!!
mcdhu

The tiller and the NWS will indeed sum their inputs ie. left rudder and right tiller will allow the aircraft to run straight ahead! This is the reason that it is very important that the two inputs are not made together especially since the deflections change with speed. In addition with there being no feedback between seats it is possible for each pilot to be countering the inputs of each other.

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.