This would be for example, where there are two practices, with one Microsoft 365 account for both practices.
It is not possible to use one single login to access two separate accounts in FYI.
The reason for this is that FYI is deeply integrated throughout the Microsoft 365 platform, including the automatic filing of emails and calendar. If there is only one email attached to multiple accounts in your practice management software, there is no differentiation of where emails are filed, etc.
A simple solution is to have another Microsoft 365 account that you can switch between.