If you open up the mail properties on the end users PC (Control Panel, Mail or within Outlook Tools, E-mail Accounts) Go to view or change exisitng e-mail accounts. Make sure 'Microsoft Exchnage Server' is selected, then select the 'Change' option.
Select 'More Settings'.
Select the 'aDvanced' tab.
You can now 'Add' the additional mailboxes you wish to view and once you have OK'd everything these
will appear in the users folder list.

The user will have to have permissions to see the additonal mailboxes, otherwise nothing will actually be displayed under the mailbox name.

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Normally you can only connect to one Exchange server at a time with Outlook. But a recently released 3rd party application name ExtraOutlook will allow you to open more than one instance of Outlook with each instance connected to a different Exchange server. Currently this 3rd party application only works with Outlook 2007 but an Outlook 2003 version is being worked on by the developers.

As long as Outlook has been around, people have been trying to get two instances running at the same time. Not multiple profiles that you can load when starting Outlook, but two separate instances running concurrently, each with their own associated profile. After all, Outlook (even 2007) only lets you connect to a single Exchange server per profile