1980 never had dual throttle cables from the factory. It was a single cable to a Keihin butterfly carb. Is your throttle tube housing aftermarket, or a newer style (84-95?) housing. It has to be to attach a throttle and an idle cable.
Most times a throttle sticks, is because:
1.) the throttle tube is sticking inside the housing.
2.) The "cruise control" shoe is rubbing or broken.
3.) The controls and throttle tube are too far onto the bar, and the end of the grip is dragging on the end of the handlebar.
Look there if you've replaced/lubes/talked nicely to the cables, and all else from the cable to the carb is fine.
I'd also just take a peep and make sure the butterfly plate isn't rubbing and sticking inside the carb, due to a worn throttle shaft or loose butterfly screws.
“When the going gets weird, the weird turn professional....” ~ Hunter S. Thompson~
R.I.P. Rubberdown. 1971-2012