Best solution -
Create two different sites in IIS and set the Host Headers in each for different domains with a different root path (c:\inetpub\site1 and c:\inetpub\site2) for example.
Workaround and easiest solution if the sites do not need to be isolated -
In your main domain, create a new default page called selector.asp which has the highest priority and copy the following:
<%
site1www = "www.site1.com"
site1 = "site1.com"
if request.servervariables("SERVER_NAME") = site1www or request.servervariables("SERVER_NAME") = site1 then
response.redirect("index.asp")
end if
site2www = "www.site2.com"
site2 = "site2.com"
if request.servervariables("SERVER_NAME") = site2www or request.servervariables("SERVER_NAME") = site2 then
response.redirect("index2.asp")
end if
%>
This will look at the host header and redirect people to the correct page! Just change site1.com / Site2.com for the address!