Edit: Torger, sorry, I didn't see your reply before I posted... some thoughts in my post are redundant now...
It's really fascinating how flexible those tandem frames are (or have to be) when it comes to different captain-stoker combos. And I really admire what torger makes possible here through different setups. The given setup to me looks also extreme in the "stoker compartment". The seat post seems to be longer than the seat tube, that's a tremendous lever trying to bend the clamping area. It obviously works out for the team... I would try to get some "extra insurance" by a more massive seat clamp - not really knowing whether that actually helps...
The stoker stem is a good base for adjustments too but -with it's thorn- a possible injury risk? Finally a nice frame/bike, I like the fact that Cannondale takes some effort to smoothen the welds and the optical result of that.