For Single Application .. Multiple Schema

You Can Use API

--Shared Component
---Security Attribute
-----Database Session
-------Initialization PL/SQL Code

DECLARE

BEGIN
IF UPPER(:APP_USER) = 'A' THEN 

APEX_UTIL.SET_PARSING_SCHEMA_FOR_REQUEST 
( P_SCHEMA => 'HR_A');

ELSE 
APEX_UTIL.SET_PARSING_SCHEMA_FOR_REQUEST 
( P_SCHEMA => 'HR_B');
END IF; 

END;