There does not appear to be any indication of the geometry of the frame or fork. There are various sizes mentioned like "Large", "small" and "medium" but I didn't notice the actual measurements of the top tube, seat tube for the captain or stoker of any listed size. How large is large and how small is the small? It would also be nice to know some of the angles like the seat tube angle which impact the top tube length.

A better deal might be a complete tandem from Santana is available for $3,000 which seems low to me but here is the page. Wheels and other components included. I wonder if Santana is having trouble moving Arriva frames.

Arriva