Do not configure multiple PDCs on a single domain. The following scenario is an example of the problems involved.
Suppose a system administrator installs a computer running Windows NT Server. The computer is called \\Main_unit, which is designated during installation as the PDC of a domain called MyDomain.
Later, the system administrator shuts down \\Main_unit and turns off. Then, the system administrator installs another server, called \\Second_unit, which is also designated as the PDC for MyDomain. Because \\Main_unit is not currently on the network, the original MyDomain is not known, and the installation of \\Second_unit and creation of MyDomain proceeds without error. The two domains are not identical; they have different Security IDs.
When the system administrator turns \\Main_unit on again, the NetLogon service discovers another PDC on the network. NetLogon fails, and \\Main_unit can no longer participate in the domain.
The system administrator now has a serious problem. It is not possible to simply demote \\Main_unit from a PDC to a BDC and continue. The Security ID (SID) for \\Main_unit will not be recognized by the current PDC, \\Second_unit, and \\Main_unit cannot join MyDomain in any capacity.
This situation happens because a unique domain SID is created whenever a PDC is created. All BDCs and user accounts within the domain share this domain SID as a prefix to their own SIDs. When \\Second_unit is installed as a PDC, its SID prefix is different from the \\Main_unit prefix, and the two computers can never participate in the same domain.
The system administrator cannot change the name of \\Main_unit and rejoin MyDomain because the SID is fixed when Windows NT Server is installed. If \\Main_unit is to be the PDC of MyDomain, the system administrator must shut down both \\Main_unit and \\Second_unit, start up \\Main_unit, and then reinstall Windows NT Server on \\Second_unit, designating it as a BDC during setup.
To avoid this problem, \\Second_unit should be installed as a backup domain controller while \\Main_unit is running. Then, if \\Main_unit is taken offline, \\Second_unit can be promoted to PDC. When \\Main_unit is ready to go online again, \\Second_unit can be demoted to a BDC. The SID for \\Main_unit is recognized by \\Second_unit. When \\Main_unit is restarted, it becomes the PDC again.
(In general, it should not be necessary to designate a new PDC unless the original PDC is going to be down for a long time.)