1) Under Control Panel > Network Connections > Local Area Connection > Properties > Internet Protocol (TCP/IP) > Properties > Advanced make sure you define two IP addresses for your server hosting IIS.  For example 192.168.1.10 and 192.168.1.20.

2) Under Administrative Tools > Internet Information Services (IIS) Manager, right click on the first web site and choose Properties.  Under the Web Site tab > IP Address, change from (All Unassigned) to the first IP address (for example 192.168.1.10).

3) Under Administrative Tools > Internet Information Services (IIS) Manager, right click on the first web site and choose Properties.  Under the Web Site tab > IP Address, change from (All Unassigned) to the second IP address (for example 192.168.1.20).

You no longer need the "host header value" defined in the sites.  But you need to update your DNS server so the correct IP addresses will be resolved.