Corbin Dunn, a computer programmer and mountain unicyclist, built this two-person unicycle. Alas, he lost a blog post about the construction of it, but his site filled with interesting pictures of his many unusual hobbies.
Link via Make | Photo: Corbin Dunn
UPDATE 12/14/09: Corbin Dunn has put up two posts about his marvelous invention.
Riding: http://www.corbinstreehouse.com/blog/2009/12/tandem-unicycle-video/
Building: http://www.corbinstreehouse.com/blog/2009/12/building-a-tandem-unicycle/
--corbin