I used the Sony Smartwatch 3 for workouts, as it has a built in gps so you dont need the phone.
Also the Huawei watch 1gen as a nice office watch.
The deal with it is that you first have to sync the first watch with google fit. (Just open google fit app both on phone and on watch, and wait a few secs) Then you turn off the first watch, and connect the second watch, and sync that with google fit and then you can leave your phone behind, do your exercise, then when you come back make sure to manually sync the workout with google fit before turning the watch off and replacing it with other watch.
You maybe dont even have to disconnect the first watch, but you must do the manual sync every time before you turn the second watch off after exercise to make sure that the workout makes it way to the phone. So that looks like this probably (take off your first watch, turn on the second watch [android wear supports multiple watches at the same time] then open google fit on phone and watch 2, do your exercise, open google fit on both phone and watch 2, wait until workout is visible on phone, then turn off watch 2 and put on watch 1)
If the explanation was a bit messy you can ask me, just don't forget to tag me @Some Random Member , so i can see it.