We have had very good luck with training on the tandem using HRMs, since pedal PMs are way too much money for our level. Stoker has a Garmin Edge because I sure as heck don't want to navigate. I have a Cateye Stealth 50. My HRM strap is paired with both the Cateye and the Edge. Stoker uses a Polar HRM, but could also use an ANT+ compatible unit and pair with that. So I see my HR. Stoker sees both my HR and her HR. She matches HRs according to some formula, plus she feels a some changes in the pedals as I initiate an acceleration or deceleration which helps, because of course the HRMs lag power. Thus we can transfer what we see on our trainers or singles to the tandem on the road and everybody's happy, which is the real issue.