Okay, so you need certain people to be able to access more than one mailbox (ie their own, and an extra one as well)?
Leave the person with their own NT logon, with their own MAPI profile pointed to their own mailbox. Then, on the mailbox you also want that person to be able to access, add their NT logon to the Permissions tab of the mailbox (if you can't see a Permissions tab for the mailboxes you need to tick the option 'show a permissions page for all objects' in the Tools Options setting of your Exchange Admin).
That way, the user can then add the extra mailbox to their existing MAPI profile (in Outlook, Tools, Services, highlight Microsoft Exchange Server, click Properties, Advanced tab, Add button, select additional mailbox to add, then OK as many times as you need to close it) and access both mailboxes from Outlook.