I’m just a volunteer here and not privy to all the Fitbit algorithms so cannot give you a specific answer as to exactly how the data will be “compromised.” Hopefully someone will step in with more information than what is below from the Help page. But the answer in the Help page (below) specifically recommends you not wear more than one device at a time.
Let me tell you my trick, even though it does not answer your question. But it addresses your situation. Like you, I’ve noticed that there are time when my watch device does not track steps accurately. (Pushing a shopping cart, anything where the arms are not in motion). When I run into these situations, I move my device to a pocket or to a purse that rests on my hip. It counts steps nicely there and when I am done, I put it back on. Steps are seamlessly counted. This will skew your heart rate info but depending on the device, you can always turn HR off when it’s not on your wrist.
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From the Help pages ... If you have more than one device paired to the same account and you forget to take one off before putting on another, we’ll reconcile the overlapping data to present the most accurate information possible on your dashboard. For example, if one of your trackers is using GPS and another is not, your dashboard chooses the GPS data when calculating total distance since GPS provides a more accurate distance measurement than step counting alone. However, the more overlapping data that exists on your account, the more likely it is that your dashboard's accuracy will suffer. For best results, don't wear more than one device at a time and sync each device before taking it off and putting on another.
