2 webservere bag 1 ip

You can't split like that. you can only forward port 80 to 1 IP.

If the reason you need a second web server is performance, then you might consider using NLB.
NLB stands for network load balancing.
Basically you have all sites on both servers and 2 NICs on each server, 1 with a unique IP and one with a shared IP.
You then forward your port 80 to the shared IP.
You need to allow the NLB feature in windows and you need the same host header name for the 2 identical sites.

You also need to make sure the sites are consistent.

I use it on some sites with 4 servers replying on 1 IP.
The load distribution does not have to be equal.

Here is a few link:
http://support.microsoft.com/default.aspx?scid=kb;en-us;256910
http://support.microsoft.com/default.aspx?scid=kb;en-us;240997
http://support.microsoft.com/kb/219277/EN-US/

Here's a good faq about NLB
http://www.microsoft.com/technet/prodtechnol/windowsserver2003/technologies/clustering/nlbfaq.mspx

Det drejer sig ikke om preformance.
Det handler om at jeg har et program hvor der er en webside knyttet til. Dette program og webside skal kører alene på den ene server. Jeg ønsker ikke at blande denne server/webside med de andre som jeg har. Jeg har serverkraft nok men ønsker at adskille de 2 servere.

Just to be creative, did you try to use the 1st web server as a front end server sort of speak?
Meaning you create a site just like on the 2nd web server and use virtual directories to the real folders location.
Which application is it if I may ask?

Det er et fortoldningssystem og ønsket/målet/kravet er at der ikke blandes andre websider eller applicationer ind i det - dvs serveren skal helst være isoleret fra alle andre ting som der rodes med.

Ja, der er desværre ikke rigtigt andet at gøre end det du allerede har gjort ! Evt. kan du prøve med en anden port end 81 !

Det var hvad jeg frygtede. Har nu bestilt en pakke med 8 ip-adresser da det SKAL køre hver for sig.
Lukker spørgsmål. Du får halvdelen af points dr.m for et godt forsøg ;-)

tak :)