Actual numbers: look for a very long rear top tube - 27 to 29" long. ------------------
I forgot to mention.... the road tandems will probably all be longer than MTB, and I have a Meridian, which is one of the longest with around 29" for the stoker. good for a big stoker.
With all due respect: MTB tandems are just as long as road tandems. Our 1994 Cannondale is 30" in the back and so will be our new tandem.