We're in the midst of preparing for a migration, that's going from Kerio with Active Exchange to Office 365. The tricky part is that Office 365 is already in use in a lot of cases, so a bit like changing the tires on a car driving down the road.
As we do the migration, what would be useful, is to have users in Outlook for Mac desktop, logged into both their Exchange account on Kerio, and their Exchange account on O365. The servers would be different, but the email address, and [depending on the situation] the UPN (or account name in Kerio's case) could be the same between the two Exchange accounts.
We're finding issues, however, with this setup, especially when related to shared mailboxes (which the UPN is a member of) and more. Let me give an example.