troubleshooting Question

Exchange 5.5 Exchange 2k3. Different Domains, Same Namespace

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gthmpd asked on
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I apologize if this gets too confusing so here goes:  
History: We have 2 NT4 Domains, Domain A and Domain B.  Each one has an exchange 5.5 site\server in one Organizational Unit.  All email flows through the site\server in Domain A.  Because of many, many flawed uses of the PDC's in both domains, we couldn't upgrade either domain to AD.  We now have a 3rd AD Domain, Domain C.  All 3 domains are setup to trust each other.  All the users have been migrated to Domain C, however their mailboxes still reside in the NT4 Domains.
We are installing Exchange 03.  At one point we tried using the 5.5 to 03 Migration method.  3 email-less days later we got our crashed 5.5 server restored. That's not an option.   Is there any scenario where we can have a fresh install of 03 installed in the new domain run side by side with the 5.5 setup in the old domains, and both have the same namespace: @blah.com?  Our inbound email flows through a Barracuda 300 and outbound through a Barracuda 200. I've read where you can list more than one server on the Barracuda.  A thought I had was having the email delivered to both servers and suppressing NDR's if the address doesn't exist while I exmerge the mailboxes over.  Is that possible?    Im lucky in that I have about a 3 day window (long weekend) where I can have email down.  My last thought was installing 03 in the new domain, having all new email stay cached on the Barracuda and exmerging all the email from both old servers and importing into the new server, about 130gs, then pointing the Barracuda to the new server. Im fairly green here so am open to any suggestions.

Thanks in Advance
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