From Can I use more than one Fitbit device with the same account?
Note that you can't connect multiple devices that use the Fitbit App Gallery to one account. For Fitbit Charge 3, Fitbit Charge 4, Fitbit Ionic, Fitbit Inspire series, and Fitbit Versa series, only connect one of these devices to your account at a time.
That tells you what cannot be; so I guess would have to be an older discontinued model.