Ok, I am posting this just for fun. Gizmodo reports that a Corbin Dunn has developed a tandem unicycle - two unicycles, one wheel – it does not look easy to ride! You can read the article, see the video and download the construction instructions – if you like doubling your fun ...
http://www.gizmodo.com.au/2009/12/unicycle-built-for-two-cool-design-an…