Undo giving Admin permissions to the users.
Give the new user rights to view the other user’s inbox from within Active Directory. If needed, also provide permissions to send on behalf of the other user.
Now have the new user set up their Outlook MAPI profile to log onto a secondary mailbox. Alternatively, the new user can set a second MAPI profile to log onto the mailbox as ‘primary’ separately.
This way the user will see both mailboxes.