How to create a publishing rule in Watchguard firewall for Legacy Exchange where The external interface IP address is natted to the new Exchange 2010 IP address

I am in the process of upgrading from Exchange 2003 to Exchange 2010 in a co-existence scenario over a limited time. My Watchguard firewall is currently natted to Exchange 2003 internal IP address. Only one External IP address can be natted to an internal address. After I nat it to the new Exchange 2010 internal IP address, how can I publish the Legacy Exchange in my Firewall?
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If you intend to publish both servers youll need 2 external ips and an fqdn that resolve to each ip.

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