It sounds like you're getting close, but again you don't need to use a separate port. For the other binding, use the same port but put in a host name like www.mysite.com (or whatever you want to use for the first site). Then, in mojoPortal, you go to Administration, Site List, Click on the Site Settings link for the site you're setting up, then click the Host Name Mapping tab. For site 1, put in www.mysite.com. For site 2, put in www.cheeseburger.com.
In your local hosts file, you should have entries for both www.mysite.com and www.cheeseburger.com, both pointing to the IP address of your web server.
As an alternative, in IIS you could also just use a single binding with no host name listed. That would make ALL incoming http connections on that port go to mojoPortal, even if you connect by IP address.
Jamie