There is a myth about SBS stating that SBS does not support additional domain controllers. That is completely untrue.  An SBS domain can have multiple domain controllers.  The onlly requirement is that the SBS server holds the FSMO roles. 

So for the record "You can have multiple domain controllers in your SBS domain".

Tools that you can use to check/transfer the FSMO roles include: netdom, ntdsutil, replmon, AD snap-ins to name a few.

P.S.  We even have a way for you to add SBS to an existing domain if necessary.

Happy New Year!