You can pair more than one Apple Watch with your iPhone and easily switch from one watch to another.
You can pair another Apple Watch in the same way you paired your first one. Bring your iPhone near your Apple Watch, wait for the Apple Watch pairing screen to appear on your iPhone, then tap Pair. Or follow these steps:
Go to the Apple Watch app on your iPhone.
Tap My Watch, then tap All Watches at the top of the screen.
Tap Add Watch, then follow the onscreen instructions.
See the Apple Support article