Tandems use the same cassettes as every other road or mountain bike, they just needs to be compatible with the spline pattern on the hub's cassette carrier. I'm guessing that a bit more than 98% of production bikes use a Shimano compatible cassette hub so chances are your hub most likely uses a Shimano spline pattern.... just not that familiar with Atomic hubs. Assuming it's a Shimano-compatible hub, you really just need to make sure you've got the right number of cogs, e.g., 7, 8, 9, or 10, to work with your shifters and rear derailleur.
All that said, in the larger cassettes like the 32t or 34t models Shimano's LX cassettes have proven to be about the most durable for many of us who ride off-road tandems. The more expensive XT/XTR models can fold or come apart under heavy tandem drive train loads. SRAMs MTB cassettes are also pretty durable.
In the smaller, 12x23-27t cassettes almost any of the better quality cassettes will work and hold up the rigors of tandem loads.
Last edited by TandemGeek; 03-06-07 at 11:32 AM.