We do not have a case designed specifically for tandems. The cases we do have are 26"x26"x10" and were designed to hold an entire single bike yet still meet the maximum regular airline luggage size requirements.
The backpack case or two hard cases (see update below) will work with most tandems but consumers should be careful to discuss with their builder the type and number of cases they intend to use before purchasing a tandem. The case you use could impact the placement of the couplings (where the frame comes apart and how big the individual frame pieces are). The standard backpack case has enough flexibility to allow some complete tandems including wheels to be packed in one case. Large tandems and some methods of frame separation may make additional disassembly (remove forks or chain ring) required to fit it into one case. In some situations an individual frame section may be too large to fit in any of our bike cases, even with extra disassembly.
Packing may require some initial trial and error to determine the best packing method to use for your particular tandem. Some builders and retailers sell the tandem already packed in it's case or cases which greatly simplifies the process.
Using two backpack or hard cases makes the job of packing much easier and allows items such as helmets, shoes, and other cycling gear to fit in the case with the bike. There will also be room to pack articles of clothing (protected by plastic bags or stuff sacks) to be packed around the bike. The more soft things you use as packing the better. The large pockets on the outside of the backpack case provide additional storage and should filled with soft items to get additional protection for the bike on the face of the case. Due to the stiffness of our hard case, it is normally necessary to use two hard cases (see update below) for packing a tandem. Using two cases seems to be the most popular and practical solution when using either type case since clothing and cycling accessories would require additional luggage anyway and this way the packing job is much easier and the bike is better protected.
Another option is to use a full size single bike case. Using two S and S cases instead of a full size single bike case makes more sense because with a tandem two people are traveling and therefore it is possible to take two S and S cases as regular luggage instead of a full size single bike case which frequently incurs an extra charge. Two smaller cases are also easier to handle and transport than a full size single bike case.
We now have hard cases available that are 12" or 14" thick instead of the regular 10" thick. These thicker cases do not meet the airlines 62 inch L"+W"+T" regular luggage size requirement (they are 2" or 4" over the limit) but they do accommodate a tandem more easily than our regular size hard case.