Talking of Tandems.....one of the 'fun' aspects of RRTY is way you can personalise it as an extra challenge. Mike mentions several schemes on the Peak Audax website.
After conventional solo RRTYs I picked off a Trike round in 2006, a MTB (with slicks) in 2008, 2 Trike rounds in 2008/9, a third solo/trike round in 2008/9, an event 'Thames and Avon 200' round in 2009. This year I might pick off an AAARRTY (with the opportunity of an event round with my 'Stroud 5 Valleys 50km' 1AAA). I cannot see me doing a Tandem or Fixie round though. The consecutive challenge is probably the hardest mentally, as it relies on so many factors; I do not know who holds the 'record' but my series runs from Jul 03 to Jan 10 and still going.
SteveP