I don't want to sound like a hardarse.....
But riding a bike is not supposed to be the same as sitting in a Lazyboy. It's more akin to riding a horse. Some bumping, thumping, standing and general major-muscle-group activities are involved. And that is part of the reason it is fun! Moreover, there is an aclimation process for those who are new to the activity, just like any other activity. Once one aclimates, a lot of the stuff that was at one time 'uncomfortable' is no longer an issue. I am willing to bet that most teams that last a year on the tandem ditch the stoker's sprung seatpost.
I would think the Cannondale with the standard sprung post and/or sprung saddle would suffice.
However, If you and/or your stoker absolutely can't stand to sit on a bike seat attached to a semi-ridged frame, you may want to consider a recumbent tandem.