We discussed these some time ago: Buddy Bike
The last great gasp for the sociables was several years ago and the vast majority of those ended up going to beach rental business in the US, Canada, Australia and Europe, as best as I can recall. These were 26" fat-tire / MTB ish models that bore the name "Buddy Bike", a moniker that has since been applied to yet another unconventional two-seat bike that was originally called the "Love Bike".
There are also a number of them floating around in private collections or that were picked up off of Ebay for a couple hundred bucks and Larry Black may still have one in his collection at Mt. Airy Cyclery. The following are photos of the more recent versions.
There's actually a pretty good thread under a series of photos taken of a home-built Sociable Bike in NY that's worth checking out: http://www.citynoise.org/article/917
I haven't read all the way through, but I suspect there may be some leads in there.
Again, the darn things will pop up on Ebay now and again. Turquoise and Yellow seemed to be the two most prevailent colors for the most recent attempt at a production run. They can also be home built in any one of a number of different configurations.
There are also a number of recumbent sociables (which is what I believe Pista Largo may have ridden) and you'll also find a few "double bikes" where essentially two bikes have simply been conjoined.
You can find photos & info on several other incarnations here: http://www.geocities.com/rcgilmore3/tandem.htm
Last edited by TandemGeek; 08-06-07 at 07:29 AM.