i've ridden all sorts of frames, singles, that is: steel, carbon/aluminum, full carbon, scandium, titanium. i'm so small, that any 50cm frame is going to be pretty stiff, and pretty light. i just worry about the durability of a carbon tandem. that's a whole lotta money down the drain in a crash. my current race bike is a ti lightspeed. i love it, and i don't worry about scratches or damage in crashes [although i've not gone down on it--yet].
racing on a carbon long bike could be a pretty risky thing. touring and fast riding is one thing, but mixing it up in tight corners and sprints, well, i believe paul sherwin refers to the argie-bargie aspect of it, and i would tend to agree.
have you raced your calfee? how up-tight about it are you?