Apple designed it so that a single iPhone would be paired with a single Apple Watch.
That doesn't mean you can't use multiple Apple Watches with your iPhone if you really want to, but it does mean it'll require some extra work on your part. Basically you need to pair, un-pair, and re-pair with your iPhone every time you switch Apple Watches. Yeah, it's a pain, but if you want a workout and a dress Apple Watch, or a fancy and a casual one, we can walk you through the process!
Full story from the iMore Blog...