Cadence is calculated by FlightDeck, not picked up by a sensor. You just program in your chainring & cassette info when you set-up the FlightDeck.

Flight Deck Compatibility Chart & Tech Info is here: http://bike.shimano.com/publish/cont...ight_deck.html

For your extralong cable, try a tandem dealer like TandemsEast, etc...
http://www.tandemseast.com/parts/accessories.html