Deploying a Second Domain Controller
As mentioned earlier, you cannot use your secondary server to create another AD DS domain, whether in the same forest or in a different one. You can, however, configure it to function as a second domain controller in your existing Windows SBS 2008 domain. A second domain controller provides redundancy, for times when your primary server is offline, and if your organization has a branch office at a remote location, a second domain controller provides users at that location with local access to AD DS services.
If you are installing a domain controller that you will be moving to another location later, you should use an IP address on the local network for this procedure. Just before you shut down the computer ...