Just love this concept

[Artson]

Just love this http://bicyclespecialties.blogspot.com/2009/04/two-cranksets-two-chains.html

[jock]

Some crazy Kiwi had a custom-made dual drivetrain system on his track bike at the Commonwealth Games some time ago.

I remember a commentator stating (deadpan) something along the lines of "he should spend more time concentrating on his training than engineering!".  Gold.