I hear this all the time But you can not add a Second Domain Controller to Small Business Server, can you ?
The answer is Yes you can in Windows Small Business Server (SBS) versions 2000, 2003 and 2008
"There can be only one Windows Small Business Server 2003 in a domain. Each Windows Small Business Server server is typically connected to the Internet either directly, or via a firewall. Windows Small Business Server does not support trusts between domains; therefore, user names and resources could not be shared between those Windows Small Business Server servers. Further, Windows Small Business Server installs at the root of the Active Directory forest, and it cannot be demoted, or have the flexible single-master operation (FSMO) roles removed."
However, the second server has to be a same Server OS as SBS. Simple as that, no other "SBS" box. It can always have a backup domain controller, it’ just can’t do trusts with a second [or other domains]. We do have to have all the FSMO roles. Though you can play Trick to have SBS Sites all talking to each other.