You can only forward the ports to a single machine. If you use UPnP, server 2 will override server 1 when it notices that ports aren't correctly forwarded, then server 1 will override server 2, etc.
In the enterprise, I would make this work by either having multiple external IP addresses, each of which goes to a single machine, or a single external IP address and a reverse proxy server which determines what web server to send a request to based on IP
address, domain, etc. Or you could have both: multiple reverse proxy servers (and IP addresses), each dealing traffic to multiple web servers.
I'm not on the WHS team, I just post a lot. :)