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).