Without regard for budget...

1. If you want to get rid of as much weight as possible, consider switching out the front fork for a 1.125" Reynolds Ouzo Pro Tandem with a DuraAce or Ultegra Caliper. It's got an extra .2mm of rake (5.5 vs. 5.3) which, if you like the way your C'dale handles, won't change it all that much.

2. Before jumping to the DT hubs, give White Industries hubs a look: lower cost, lower weight, higher reliability (at least IMHO). Moreover, if you change out the fork you can cut your front hub in half.

3. Cranks aren't inexpensive if you're looking for superlight. Best bet if you want to save a few bucks is to search out a set of Ultegra tandem cranks: big bang for the buck.

4. Not sure what C'dale has spec'd for seatposts and handlebars, but if you think there are some weight savings opportunities there and can resist the lure of carbon consider Thomson's Elite or Masterpiece aluminum posts and SL-grade aluminum handlebars from Deda (e.g., 215) or others.

That's about it, aside from lighterweight tires vs. the stokers.