Ran a pair of Sigma for a while... upgraded to a pair of Garmin 510's and could not be happier. Both units share a single speed/cadence sensor in the rear. Each rider has their own heart rate monitor and readout. And the second speed/cadence sensor found a home on my half bike. The Garmins are very configurable and using different profiles on mine allows seamless operation on either bike. IMHO this is just the way to go with a tandem.