Wow. Welcome. Whoa ... that is quite the o.p. A lot to unpack there. But I will start here: it is NOT just Co-Motion or Santana. I am not knocking those brands at all, but there are other options: Cannondale, KHS, Trek, Burley and those are the mass market options. I know much less about the custom builders but there are options there too: Rodriguez? Especially since you are considering used, look at Tandem Classified's. What is your budget? Really? I respectfully suggest that, before you drop $10K on a custom Santana that you spend another season or two on a properly fitting lesser bike, learning what you really want and don't want. Yours and your stokers dimensions and what you want to do with a tandem is more important information to us than the specs of the bikes you are considering, but, as long as we are there, if it was me I would be looking more at the 3 x 10 than the 2x10! And the first thing I do to ANY triple I get my hands on is toss the 30T granny for a 26T or even 24T. On its face, the Sovereign could work just fine, but it should fit you and yours! Be careful that that dealer doesn't just want it off his hands. I am 'only' 5'10" and I am always on a LG. Even if it means getting a 10mm to 20mm shorter stem, I'll do that before considering a smaller frame. YMMV.