I use an 810 on my single and my wife uses a 500 on hers. On the tandem, my wife uses my 810 for mapping and directing us and I use her 500. The HR belts record on our own computers regardless of position on the bike so downloading is fine and I don't have to re-sync when we switch computers back on our singles.
Also, both units pick up cadence from the captain's crank.
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“Train hard until your legs are tanned, then keep going until the shape arrives.” -Jolanda Neff