let me be a little more clear...
the two web servers behind the firewall are running two different HTTP servers. So, one Apache server and one Lotus Domino server. Whatever "router/gateway" implmented needs to be able to tunnel to the other HTTP servers.
I'm looking into the virtual hosting...
I had this working in a former life running Raptor as the firewall. There was a very simple table of domain names and their corresponding IPs behind the firewall. When traffic was received requesting information for a particular domain, it would route it to the appropriate server.