How to make parameters conditionally mandatory or non mandatory from Concurrent Program Application

1. Create first value set with None Validation
Ex : Start Date (Parameter)
Create Second value set( End Date) with Special Validation
Click on Edit Information
Select event type as validate.
Paste the following code.

FND PLSQL "BEGIN
if ':$FLEX$.XXDB_START_DATE_VS' is null then
NULL;
end if;
END;"

The above code returns false if parameter 1 is null .
If parameter 1 is not null then second parameter get enable.
Attach this two value sets to start date and end date parameters