How to merge duplicate Outlook mailboxes

Users mail in original mailbox was lost when user logged into Outlook while pc analyst was altering users profile. Now user only has mail from that point on. Exchange Admin shows both accounts each on a different server with correct different mailbox sizes but when trying to change aliases to be able to open both separately, changes instantly replicate to the other account that has only the most recent email. How can we get to the other mailbox and its contents?