|Spotted yesterday on King St, Manchester|
Note the two chainwheels located on the left hand side of the bike. This means their pedal threads have to be drilled opposite to the way that normal right hand side chainring cranks are drilled. i.e. you can't use those chainwheels on a normal bike (the pedals would unscrew).
Although it's obvious I didn't know this about tandems until I was flogged a tandem chainwheel that was advertised as a normal chainwheel on ebay last week. Luckily a refund was forthcoming so I've bought a different one from Derekscycles instead, who not only have a great shop name but provide a damn good service too.