Office 365 and multiple accounts in Outlook -- sending email from 2nd account

I have a user who, when we were still on Exchange 2007, she had two accounts in Outlook. At that time she was able to send from the 2nd account and it would go out the Outbox on the 2nd account and identify the sending email address as the 2nd account.

Now that we've moved to Office 365, both accounts show up in Outlook, but when she sends an email from the 2nd account, it always goes out the primary account with the primary account's email address.

How do I get it to work like it used to?