More appropriate , perhaps is a system approach, where a tandem has a addition/ subtraction in the design.
so the second rider's portion is added , to a single bike , to accommodate the stoker, Un bolt
the additional section, bars , drive train etc, to use it as a single bike ..
several frame builders come to mind ..R+E in Seattle WA, http://www.rodcycle.com/

and Bike Friday in Eugene OR.http://www.bikefriday.com/