Welcome to the forum!
If 2 throttle arms are moving when you move the 'D' (outside) throttle, then the throttle arms are touching internally. Unplug the transformer, remove the cover and physically look at the 2 throttle arms and see if the touch each other. If they do, then carefully bend one of the arms so that it does not touch the other. Do not bend too much, or it may crack. There should be no need to remove the arm from the transformer.
Check the carbon rollers also, and replace any that are worn.
Here is a page from the service manual to show the exploded view of the arms.
http://pictures.olsenstoy.com/cd/transfmr%5Cpszw4.pdf

Larry

Nick,
Glad to hear that you were able to fix the problem with minimum bending.
The rollers and rivets (pins) are available online and are easy to replace, exactly as you wrote. The pins were originally riveted in place with a rivet press (which can be purchased), but a gentle squeezing of the very end of the rivet with a pair of pliers will do the job. Squeeze just enough to keep the rivet from being able to be pulled out, but not so much to keep the roller from spinning.
 The rollers and pins can be replaced without removing any parts from the transformer.

Larry