Originally Posted by waynesulak

It is possible to flip a Thompson setback seatpost forward. This would compensate for a 73 degree seat tube angle on most tandems vs steeper seat tube angle on the stoker's single.

From FAQ on Thompson site:

– Can I flip my set back post around to be a “set forward” to use my road bike for triathlon?
A – Yes you may. You may need to turn the top clamp to face the front of the bike for saddle adjustment.

http://bikethomson.com/seatposts/elite/

If the stoker spends significant time on both her single and the tandem, I think it is well worth a try to get this right. A different setback changes the muscles most used while pedaling. If the stoker is going to get the consistent training the set back should be the same.