Join Date: Sep 2008
Location: Sesame Street
Bikes: Swobo Folsom, Diamond Back Master TG, Mongoose Alta, Huffy Daisy Tandem
I think on a mixte frame (and any frame for that matter), the st length is not nearly as important as the top tube length. I would personally shop for a bike that would yield a good compromise cockpit length with a given stem length and bar type for the two riders.
Honestly, unterhausen has a good point about how the bike will be a compromise for both riders, I would shop around on craigslist and the used market in general for properly-sized bikes for each rider. In the long run this would likely end up being cheaper and easier for both riders in question. After all, building up a bike (esp. with all new parts) can get incredibly expensive.