I think that you need to stand on the tandem, at least for a few strokes (20-50) as often as you do in your single. Shift your gears up so the pedals will support your weight as you stand and stroke down. As you stand, try to keep the side movement to a minimum. Ask your stoker not to fight (try to correct) the frame movement. Tell your stoker that you will get better at standing without ecxesive movement. Hope this helps.