You can always use only one authentication scheme. An authentication scheme can use multiple procedures to check if user may log in or not. The standard authentication scheme (LDAP, APEX) are designed the way that you enter some parameters in a form (LDAP IP, Port) and can use it. In your case you can't use theese built in functionalities. However, you can write a package and combine them in one and put them in an order - LDAP is checked first and if it fails, APEX built in accounts will be checked. If you know how to write PL/SQL, this shouldn't be a problem for you. I have done this before and it works.