"What are tandem worthy disc brakes?"
Here at Rodriguez, we do things a little differently. Those little differences make a big difference when it comes to something like your brakes. For 2014, we're offering a disc brake set-up that can stand up to the demands of the serious tandem team.
For many years, we found that the dis-advantages to primary disc brakes on a tandem outweighed the advantages that they offered...at least for most of our tandem folks. Well, a couple of advancements have solved one of the major dis-advantages. Though this combination does not solve all of the issues, and adds some cost to your build, it really helps to solve the problem of over heated, warped, noisy rotors that have plagued the serious tandem cyclist for the last several years. For our higher-end tandem customers who want to run disc brakes as their primary brakes, we spec the Avid BB7 mechanical disc brake caliper paired with the 200mm (largest diameter) Hope Floating rotors. The combination is the one that we feel is worthy of a true recommendation for those customers who ride a lot in the rain (and don't want their brakes to go 'sha-shingggg, sha-shingggg' as they ride).

Hope Heavy Duty Floating rotors
The Hope Heavy Duty Floating rotors have 2 steel braking surfaces attached to a machined aluminum, heat dispensing spider. The combination is costlier than a standard set-up, but is well worth it on a tandem. As an added benefit, they are also lighter than the standard rotors.

Avid BB7 Calipers
The Avid BB7 mechanical disc brake caliper is at this point the best caliper that is recommended for tandem use. In combination with the Hope Floating rotors, this caliper does a good job stopping a tandem with minimal heat issues.

Some of the disc brake calipers on the market specifically state that they are "only for use on a single rider bike". So, just be careful to use a disc caliper that is tandem recommended, or at least not specifically 'not recommended' for tandem use.

Hope Floating Disc Rotors

Avid BB7 Disc Calipers