There are/were adaptors, like Cyclemorph that will cost you around $600. cyclemorph.com
Tandemrack, a wannebe tandem system, costs about $120. You attach/detach second bike to first bike. HYPERLINK http://www.x2cycle.com
Both of these systems were at the Interbike show in 2005.
Caveat: have seen it, but not tried it.
Our opinion: try a real tandem. Learn how to ride/communicate on it. Coast when stoker asks.
Borrow, rent or purchase an inexpensive used tandem.