Assuming you mean for the same person in same account,
see Can I use more than one Fitbit device with the same account? ,
including this:
Note that you can't connect multiple devices that use the Fitbit App Gallery to one account. For Fitbit Charge 3, Fitbit Ionic, Fitbit Inspire series, and Fitbit Versa series, only connect one of these devices to your account at a time.
If you mean 2 different accounts for 2 people, it is possible, but with some inconvenience.
You can have them on same phone but then you have to keep logging out of the Fitbit app on one account, and then log back in to the other account, rather than just always being logged into one account.
So it is easier if you have some other sync device, but it can be done with just one phone.
Thanks for your help