HRMs. Have to be coded, so you don't pick up each other's. Decide on ranges before the ride, depending on how you want to train that ride, how tired, etc. We sometimes use different ranges, depending on how fresh one of us is.
The cadence non-issue will fix itself. More important is to get so you use the same power in the different parts of the stroke. Trying to accelerate someone else's feet takes a lot out of a person. It shouldn't feel like someone else is pedaling at all. So you might say, "A little more kick at the top of the stroke" or "Am I pulling too much at 8 o'clock?"