Yeah sure you can...
I'm not sure why you would want to though. Twice the resources, twice the risks, twice the amount of things to go wrong... I could go on but I won't.
Don't know the in's & out's of IIS but with Apache you could just use one server and setup a virtual host on port 81 in the vhosts config file as well as the default document root on port 80. Like so...
Theres no need to edit the windows host file at all aswell for this. If you want both sites 'live' don't forget to port forward if your machine sits behind a router, and dont forget your firewall exceptions.
Hope this helps somewhat!