R12 can still use ONE AD domain only.
The LDAP search base in R12 can be pointed to just one domain. If there are multiple domains in an infrastructure then we can point the ldap search base to the root of the ldap Hierarchy. For example if there is a parent and child domain then we can point the ldap search base to the parent domain so that ldap can search the contacts in the parent domain as well as in the child domains. In this scenario , it is recommended to use the Global catalogue port ( 3268 ) as the ldap_port for faster searches and faster ldap sync process.
CA Directory can be used for this purpose.