[otrs] Ldap authentification - several BaseDN's

Hi there! How can I combine several BaseDN's for authentification. e.g. BaseDN1: ou=b,ou=a,o=hbd BaseDN2: ou=c,ou=a,o=hbd (To search through ou=a,o=hbd is not possible, as there are other subtrees that contain the user data - 'duplicated entries found')? lg martin