How do I merge two mailboxes on the same server?

The best way is not to create a new one and just to attach the old mailbox to the new user.  What you can do if the old mailbox hasn't yet been purged, is to delete the new mailbox in the normal way you would do so.  Then, find the old mailbox in the ESM (Servername/storage group/mailbox store/mailboxes), right-click on the mailbox name and click "Reconnect." Then select the new user account to reconnect the mailbox to it.

The other option would be to export all of the contents to a PST through Exmerge and then import it into the user's new account mailbox.

Unfortunately The new account and mailbox were created a week ago and the user is already receiving mail in it so I can't delete it.

I already had the PST idea but was hoping there was some way I could do this from the server end, and not have to remote control the users machine or walk the user through any import functions. The remote computer is on a slow connection.

Considerig you are using Enterprise Edition. Also Considering OLD Mailbox was in SG1DB1

1> Add SG1DB1 in Recovery Storage Group. Mount the stores & dismount again.
2> Create New Mailbox store (may be in new Storage Group).
3> Restore offline copy Recovery Storage Group to this new store.
4> Once it is mounted - from Mailbox Store - mailbox - right click & select "Run Cleanup Agent"
5> You will find all mailboxes with Red Color Cross because their prodction mailboxes are active on other store with AD account. Sicne these account does not have AD account they are orphaned.
6> Create New AD acccount without mailbox.
7> Now use Mailbox Recovery Center to reconnect OLD mailbox to newly created AD account (For additional information about Mailbox Recovery Center, search for Mailbox Recovery Center in Microsoft Exchange Server 2003 Help - no articles available)

* Reason: When Recovery Storage Group is created it will check for AD account of that mailbox. Since OLD account is not there it will not restore using ExMerge.

vishal, you are technically correct about needing an account for the mailbox for Exmerge to work, and I suppose I should have mentioned (and intended to mention) in my post about Exmerge that you would have to reconnect the old mailbox to an account before running Exmerge.  The best thing to do would be to use a temporary account - reconnect the mailbox to that account, make sure the user account you are using to run Exmerge has full mailbox access, and then do the Exmerge process.  I still think this is quicker and easier than doing the mailbox recovery group thing, but that of course is up to you.

Thanks for the help