I have the same problem as well. I still must end this mission, but i find it easyer to use the front, towing tug to sail the whole combination, and don't use the tug that's alongside the tanker. Or just with 20% thrust and the rudders in a position that keeps the tanker a bit on course.
However, The rear tugboat has all the waypoints, so i'm thinking the following. Don't use the front tug. Leave it where you find it, and use the rear tug to tow the tanker. I think it's much easyer, and you have all the waypoints. When you delivert the tanker, switch to the tug left behind, and move to the waypoint of that tug.
Well, that's what i am going to try.