This is probably a stupid question but why is the frame size of the tandem all that important. Could a small frame work for pretty much any team. By changing stems, adding a stem riser and adjusting seats isn't it essentially possible to get a frame that works for anyone? It looks like rear top tubes are all the same and I'm thinking that's the idea with the co-motion periscopes (still offered in small and larger). I would guess the reason not to do this is control and stearing but how can you tell if it's unsafe (before you crash). What else am I missing.
I'm saying this because frames that are "too small" for us appear often while the XL and the Jumbos are a bit more rare. Read between the lines as there is a small co-motion that is a great deal but we currently ride a Cannondale Jumbo and I'm trying to convince my wife that we should get it.