I know this will only be of minimal help because I don't know the technical details, but you should be able to set up what you want with the right router. We have two sites hosted on one server and one IP address. The router passes not only the IP, but also the URL to tell the web server which site to serve. I would assume you could do the same thing with two physical servers, just having the router decide which box to pass the call to based on the URL.
Unfortunately, I can't give you more details. Our host deals with those kinds of details so I don't have to.
I also don't know if it would make a difference whether you are using a Windows or Linux server (we have Windows).
There's a bunch of ways to do this, but essentially it comes down to this:
1. Single IP = Harder 2. Multiple IP = Easier
Since your ISP doesn't block port 80 and you already have a static IP, you should call up sales and have them allocate you another, for a monthly fee of course. For example, my provider Speakeasy allocates 8 public addresses with a business ADSL account.
Since you have an ADSL router and not an ADSL modem connected to a router, you’re probably going to have to introduce additional hardware. Most consumer level routers cannot have multiple IP's on a single eth interface but I would check with the product manual or ISP that provided it to you first. You might be able to operate the router in bridge mode. You'll then need to spring for any off the shelf router and ethernet hub.