I had a Sigma BC2209 MHR on my half bike. Full wireless with speed, cadence, and HR. I picked up a second one when we got the tandem. Found that the sensors were a bit range challenged compared to a half bike but it would work. Put the cadence sensor on the captains crank and the speed sensor on front wheel (I put both on back on my half bike). Worked quite well but with one limitation: we only had capability of one HR monitor. So either I could see stoker HR or stoker could see mine.

Then one day stoker was complaining of no readings... so now we have a set of Garmin Edge 510's which use ANT+ sensor tech and should be able to give us dual HR monitors.

Turns out the stoker had not put the Sigma all the way into the mounting bracket. Fully in the bracket and readings were rock solid. But we had already purchased the Garmins at that point and had been trying to figure out dual HRM anyways.

So there are a couple ways to do it.