Read this thread... I think it addresses your question with as much or more information than you'd ever want. While it starts off talking about hubs, it quickly migrates into the hub brake discussion:
Clueless new tandem owner and rear hubs....
Short answer is:
If you want an auxiliary drag brake to help check your speed on long descents, go with the Arai drum brake; it's the only drag brake available.
If you want to replace your primary rear rim brake with a primary hub brake, the Avid ball bearing mechanical disc brake is the one that seems to be having the greatest amount of success; however, Avid does not warranty nor approve their use on tandems. Despite this minor detail, Co-Motion, Erickson, daVinci and other tandem builders are now routinely installing them as primary brakes and to my knowledge there have not been any reported problems. Your drum-ready hub can be fitted with an Edco, thread-on disc adapter that should allow it to work with any 6 bolt disc rotor so you wouldn't necessarily need a new hub. If you have an RT model Cannondale tandem with STI Levers be sure to specify the Road model of Avid disc brake. If you have the MT model Cannondale spec. the MTB model.
Best bet for making sure you get a system that will bolt-up right the first time is to contact one of the tandem-only dealers who sell Cannondale tandems, e.g., pay a visit to www.TandemsEast.com
(ask for Mel) or www.TandemsLtd.com
(ask for Jack).
Please keep in mind, using disc brakes on road tandems is in its infancy and there are still a lot of "unknowns" with regard to long-term reliability and durability. Be sure to be attentive to rear brake maintenance and periodically check your pads for excess wear and tear. Always keep a new, spare set of pads in your seat pack "just in case".
Last edited by TandemGeek; 01-30-05 at 08:54 PM.