LDAP RTC 6.0.1 Map multiple Organizations Units on base user search

You mean the "Base User DN", right? The easiest way to do so is to use the DN one level up. For example, if you want trying to use "ou=vendors,dc=mycompany,dc=com" and "ou=contractors,dc=mycompany,dc=com" at the same time, simply use "dc=mycompany,dc=com" instead.

Also keep in mind that for LDAP configuration there are two places where you would configure the "Base User DN" - the application server (WAS/Tomcat/Libery), and JTS (Advanced Properties).