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