My son and I have shared his watch for a few days whilst I wait for mine and see if it's worth having.. I'm assuming it does as none of his data was visible on it once I paired it to my phone. Also I this second (with great regret) unpaired it from my phone to give back and it goes through a process of deleting all the data and powering up like you just look it out of the box.. Asking for language etc.. So I'd unpair, rather than delete all, which should remove the association from both sides..
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One other thing I noticed it takes a good 10 minutes to do which makes the prospect of owning two watches quite painful if you wanted to swap between a sport and a Stainless Steel.... Maybe in a future app they will let you have two watches synced to the phone.. That'd be nice!