For consumer routers, you don't. For most SOHO routers, likewise. To forward port 80 to two different devices depending on domain name (or other criteria) you need a reverse proxy server, which is not part of Windows Home Server (or a home or SOHO network
for that matter).
You can achieve something similar if your router supports it (please consult your router's manuals, or the manufacturer's technical support) by forwarding a different port to one device on port 80. Your router will have to be able to
map a port for both incoming and outgoing packets, however, and again most consumer routers don't do this. There are also usability consequences to doing this: your users will need to remember that for
this device, they don't need a port, while for that one, they do.
I'm not on the WHS team, I just post a lot. :)