Tandem bents have been offered by....
Angletech Cycles, Barcroft Cycles, BikeE, Bilenky, Cambie Cycles, Flevobike, Greenspeed, Hase Spezialräder, Kettwiesel tandems ICE, M5, Linear, Longbikes,Optima, Organic Engines, Ostrad, Rans, Ryan, S & B Recumbents, Speedbike,Terry Osell, Varna, Vision, and Zox (who have I forgoten?)
Some of the builders listed have stopped offering tandems or closed shop. But they still come up for sale used from time to time. And some of the builders listed only offer tandem trikes.
Angletech Cycles: Offers custom built high end models of other builders frames. *Barcroft Cycles: SWB/ASS tandem. Good build quality. Rides well, good sport bike.
*BikeE: Belly up
Bilenky: Custom built only.
Bolder Bikes: Custom built
Cambie Cycles: LWB/ABS tandem
Flevobike: Back to back
*Greenspeed: The tandem trike to have
Hase Spezialräder: Back to back
Kettwiesel: Really not a tandem to single trikes hooked together to make a tandem
ICE: Trike (nicer then Greenspeed IMHO)
M5: Back to back
*Linear: Belly up, but Bicycle Man has bought the line is offering singles, no tandems at this time
*Longbikes: SWB/USS and LWB/USS or ASS
Optima: No info
*Organic Engines: US built tandem trike (about 1/2 the cost of a Greenspeed)
*Ryan: No longer building bikes.
S & B Recumbents. LWB/ASS
Speedbike: No info
Terry Osell: Custom builder, may have retired.
Turrner: Three wheels all in line, two in front steer the bike
Varna: No info
*Vision: Belly up SWB/USS mono tube
Zox: SWB/ASS box tube frame
*= Bikes I have owned or riden
DO you know what kind of tandem you want SWB LWB USS or ASS? Bicycle or trike?