You need to configure each website to respond to the correct host header. This will enable you to serve both sites on the same port on the same server. For example www.domain1.com and www.domain2.com. If you only have one domain, you can specify a different host name (e.g. site1.domain.com and site2.domain.com).
In IIS7 you set the host name in the site bindings dialog box.
Step by step instructions:
Open Control Panel
Open Administrative Tools
Launch Internet Information Services (IIS) Manager (Note: Do not open the 6.0 Manager.)
In the connections pane, twirl open the tree until you see the website you've created previously
Select the site
In the Actions pane, click the Bindings link
Select the binding you want to edit
Click on the Edit button
Add the appropriate host name (e.g. site.domain.com; do not include http:// )
Click on OK, then Close