use host headers

its two different servers though? wouldn't that make a difference?

I don't know how to do that with two different servers.....

You would have to use a different external address.

DNS point example1.example.com to 1.2.3.4 and any packet on port 80 is forwarded to IIS1
what you want to do is also
DNS point example2.example.com to 1.2.3.4 and any packet on port 80 is forwarded to IIS2.

Something somewhere has to uniquely identify which server the request is destined for
1) Hostheader - I can't see how that would work across 2 servers
2) target port number - I can see why you wouldn't want to use something other than port 80
3) targetr address - easy way - assuming you have more than 1 public address.

4) Move both sites onto the 1 IIS server and use host headers

5) Use a load balancer or FE IIS server to differentiate between requests on a host header basis and pass off to IIS1 and IIS2

Take Care

Matt
I have always wished that my computer would be as easy to use as my telephone.
My wish has come true. I no longer know how to use my telephone.

Yeah, I was going to say - a loadbalancer would do this.
 
Or, one of your webservers could run as a reverse proxy.