Usually when a doubleheaded train ran on the NG (dispite where and when the second locomotive was in the train) it was referred to "Extra 489-499 West", and went on the call book and train order as such. There are times when an actual helper was called and was noted as such in the call book and train order. Generally a helper on the west end would be called from Durango to run to Gato and help the train west over Florida Mesa.
I don't know about the pay differential, I'll have ask the experts about that.