Synchronize Calendar between 2 mailboxes

hi,

here is the situation.
the manager and the secretary are both using outlook 2007. each one has an exchange mailbox (no pst is used).
in the manager's mailbox, i gave owner permission to the secretary for the calendar and other folders such as inbox and sent items.
i added the manager's mailbox to the secretary's outlook and she can access it and modify it with no problem.

Now she wants to synchronize her calendar with his calendar. whenever she accepts an appointment, it should automatically appear in both calendars.

How can i do this?