This was on the page that I linked to and you didn't read;
Switch between watches
Only one Apple Watch can be active at a time. To switch, take off your current watch, put on another one, then raise your wrist or move your arm. watchOS doesn't support wearing two watches at once, and it doesn't support sharing watches between multiple users at the same time.
Your watches share Activity and Workout information, so your progress automatically updates after you switch. If you put on a different watch to work out, you can still see all your steps, stand credit and exercise credit for the day. Depending on how much information needs to sync, it might take a minute for you to see all your progress.