This would be for example, where there are two practices, with one Office 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 Office 365 platform, including the automatic filing of emails and calendar. If there is only one email attached to multiple Xero Practice Manager accounts, there is no differentiation of where emails are filed, etc.
A simple solution is to have another Office 365 account that you can switch between.