The best way to know which is the active Apple Watch is to change the name of your watch.
- Launch the Watch App.
- Select General -> About -> Name
- Change the name of your Apple Watch.
- Click Done
- Click General -> My Watch (at top left navigation)
- You should be back on the main screen. The name of your watch should have changed.
- Force close our app
- Launch our app
- Go to the Fields tab
- You should now see the Apple Watch with the new name. Select it as the source for steps.
When you rename a watch, Apple Health considers it brand new. In order for it to show up in the Fields tab, there has to be some data on it. Turn our app off, launch the activity app, walking around a little bit with your watch, and then turn our app back on and it should show up.