In the old days, you could. However, when Fitbit switched to a new O/S (an enhanced version of the Pebble O/S) this was no longer possible. Pebble only had one device so they didn't need the ability to support multiple devices on a single account so the O/S was not designed for it. This is an O/S designed for smartwatch features. I don't know if it is technically possible to change this, but you can make suggestions for new features here.
