Hi
It is not necessary to unpair your original Apple Watch in order to be able to pair and set up a second Apple Watch with the same content and settings.
Historic fitness / Activity data is stored on your iPhone, not on your Apple Watch.
After pairing a second Apple Watch to your iPhone, Activity data for the current day is synced automatically from one watch to the other, as and when you switch between wearing them. (Note that only one watch can be active at any given time).
When pairing and setting up your second Apple Watch, if you would prefer to configure it with the same content and settings as your original watch, choose to Restore from Backup during setup (content and settings from your original watch is backed up to your iPhone automatically). Otherwise, set it up as a new watch.
More information for using more than one Apple Watch with your iPhone - including for switching between them - is available here:
Use more than one Apple Watch with your iPhone - Apple Support