The obvious first choice is to copy an existing design. Or at least taking solid cues from a few. Having over 35,000 miles on a couple of tandems and seeing a LOT more up close I like the CoMotion design style for steering geometry, especially for experienced tandem teams. The front triangle is almost like the captain's single, maybe a tad shorter in top tube. The stoker's mid section can't be too long by some opinions. One thing to consider is the BB c-c length, timing chain adjustment/ring size, eccentric range relationship. There's boom tube length steps that work better then the lengths between the steps. For the rear triangle, again longer is better. Tandems stretch gear range limits often and with the drivetrain so far away from the captain the likelihood of cross chaining is much stronger. A few more CMs of chainstay won't make much difference in the handling but might make the stoker happier... Remember to keep the stand over clearance large. Andy.