One reason that I know affects me is the size differences between captain and stoker. I am 6'2" and my stoker is 5'4". In order for her to be comfortable and able to stand over the bike, I have to go a size or two smaller than I would on my single. Just for reference my single is a 63cm and hers is around a 48-50cm. From what I have looked at, short of some kind of special setup, it is easiest for me to raise the seat and put a longer stem on a smaller frame. The only other option would be to select a bike where she cannot stand over it comfortably, which isn't going to happen. Just my thoughts.