1.This is not a best practice and nobody will recommend this for DC's
Build each server as Windows 2003 and promote them to be domain controllers. Active Directory automatically replicates with other domain controllers in the same domain and provides complete fault tolerance (with the exception of FSMO roles).
Make sure you install DNS on both servers and created your DNS zones as AD integreated. Also ensure you make both DC's global catalogs.
Windows AD objects SIDs for a variety of things. You cannot have two servers with SIDs that are the same and you cannot have two servers with the same name without creating problems on the network
Ref this link....
http://support.microsoft.com/kb/281662http://www.experts-exchange.com/Software/Server_Software/File_Servers/Active_Directory/Q_23513942.html