Upgrade to CUCM 6.x! I believe you can only sync or integrate with (1) AD domain with 4.x. It's a schema update that works with only (1) forest.
Your other option is to build out a separate domain C that is empty. Sync Domain A and B to the Domain C.
CUCM would then sync with Domain C.
In CUCM 6.x, you can integrate multiple LDAP directories. (but only use one for authentication remember)