Is it possible to Link two user accounts to the same exchange mailbox ? (exchange 2003)
No, but you can give other users the rights to access it and/or send messages as the owner.
Not that I'm aware of i.e. in Active Directory/Exchange there is a one to one mapping between user accounts and exchange mailboxes
But you can give more than one user access to the same mailbox. So that if your aim is to allow multiple users to view and interact with the mail that is possible.
In Outlook the user will now see Mailbox - [User], Any PST archive and Mailbox - MyResource
This keeps that mailbox separate from their own mail. If you just want people to see inbound mail and you don't mind about duplication of effort you can create a distribution group with the email address MyResource@domain.com but UserA still sees a mail as unread even if UserB has actioned it.