If you go into the first site, in Administration Menu > Site Settings, you should see a dropdown next to the site title, choose the 2nd site from the list then on the Host Name mapping tab specify the host name domain2.xxx.com
Any host name that resolves to the ip address the site is running on will always use the first site unless it is mapped to a different site by host name mapping. So its really only needed to specify it for the secondary host names.
Hope it helps,
Joe