I tried riding in the stoker position one time...it didn't go well. We couldn't keep a straight line. I figure it was an issue with me in the stoker position.
We started on a tandem but my wife rides a single now. When she rides her single there is hardly any side to side movement of the bike even when she is out of the saddle. Basically I think the stoker needs to focus on staying steady a lot more on the tandem than the captain (no scientific basis for this statement). If they are swinging the bike or steering the bike it responds.
It's a delicate thing to tell someone who has no control that they need to stop moving and trust. I know I couldn't when I was sitting in the back seat.