In migrating Email accounts in Outlook 2010 to Office 365, Ive been copying email from the users existing exchange account to the new Office 365 folders (after adding the new Office 365 account as an additional Exchange account in Outlook)
I then needed to add additional mailboxes from the new Office 365 users onto the secretaries Outlook, however the normal functionality wouldnt work. Additionally when I tried to remove the existing Exchange account from Outlook after copying all the emails and folders over, I found that Outlook wouldnt let me.
The only solution seems to be to create a brand new Outlook mail Profile for the new Office 365 user. Then you will be able to add the new Office 365 additional users.
The reason is that Outlook doesnt allow opening multiple mailboxes that belown to different Exchange organisations or domains.
See this thread for more information