Migrating an exchange 2003 user mailbox to another user without using exmerge.

Have a user mailbox that is extremely large and the user is an executive Manager that doesn't want to archive mail.  As an interim solution until we migrate to Exchange 2010, I'd like to create a new 'account' and migrate all of the mail from the 'primary' account mailbox to the 'secondary' mailbox.  The intent being that the user will connect to both 'mailboxes' in Outlook.  This will allow faster processing on the 'primary' mailbox and still allow the user to connect to all old mail, etc.  I tried deleting the mailboxes and then 'reconnecting' them to each other's 'account' in the hopes of doing it quickly without using exmerge. Because I think exmerge is going to take about a day and a half and the user doesn't want to be without mail for that long, etc.

Wondering if anyone has tried anything similar or has an idea about another way of accomplishing this.

Thanks!