I'd be concerned about two things:
1) Stem clamp. Even if the stem were well-deburred, some CF posts have only a limited range where they're supposed to be clamped (AL insert on limited length inside).
2) All the forces a captain's seatpost undergoes - more than a half bike.
And I've heard stories of catastrophic handlebar failures, so I'd be wary with CF under the captain. After all, if the captain falls off, stoker is sure to follow!
But that said, I'd also be curious if this is for road or mountain. I hear Stapfam has bend many seatposts on his mt. tandem. I don't see a response from him; I'd like to hear what he has to say.