+1 on Cannondale being a great value, especially used. Another plus is _knowing_ that the frame is stiff enough to be steady at speed so no worries about whippy handling on high speed downhills. If looking used, IMHO, the watershed points are 145mm rear dropout spacing and threadless headsets. A tandem with both of these will be reasonably modern in its componentry, and can be upgraded to any other new components you would wish. Of the two, I'd take a threaded headset before a narrower rear dropout spacing because the spacing impacts the whole drivetrain.
Also, definitely read the link cajoe posted together with your stoker and talk about it -- communication is key!
Last edited by WheelsNT; 05-25-12 at 09:03 PM.