Issue while implementing Validations in Dynamic Selection Screen

Hello ABAP Gurus ,

I have a Dynamic Selection screen ,

If I press Rad button BP Based then below selection screen will come

and If I press Rad button CA Based then below selection screen will come

Now in Both Screen Blocks Billing Period field is common.

I have to Keep that mandatory which is a PARAMETERS type declaration.

Now issues which I am facing :

1. If I use Obligatory then

the screen wouldn't allow a user to Select second radio button reason being first radio button's Billing Period is also obligatory and its empty, similarly vice versa.

2. Now am using a FLAG which will get SET when Individual Screen gets active based on Radio Button click ,

and based on that flag am writing my code in AT SELECTION SCREEN even that is not working properly.

Reason being , At very first initial run AT SELECTION-SCREEN OUTPUT will trigger and it will set flag = 'X'.

and selection screen appears , now the user selects Second Radio Button and at that time AT SELECTION SCREEN event gets triggered and its shows the message for First Screen Blocks Billing Period only.

Please guide for this issue.