The router cannot dispatch the HTTP GET requests for you; the most it can do is dispatch based on the port. Try passing all requests on to your Apache. You should be able to configure the Apache to forward part of the requests to your other webserver.
Any particular reason why both must be externally accessed via port 80? If not, just give them distinct external ports X and Y and set up the port mappings:
X -> Apache machine : port 80
Y -> IIS machine : port 80