Hello Dirk,
I modified your code a lil' bit & it works for me 
SELECTION-SCREEN BEGIN OF BLOCK b1 WITH FRAME. PARAMETERS: all_dsos RADIOBUTTON GROUP ip USER-COMMAND psas, all_psas RADIOBUTTON GROUP ip . SELECTION-SCREEN END OF BLOCK b1. SELECTION-SCREEN BEGIN OF BLOCK b2 WITH FRAME . PARAMETERS: f_dsos RADIOBUTTON GROUP ds MODIF ID ds, i_dsos RADIOBUTTON GROUP ds MODIF ID ds. PARAMETERS dso_ia TYPE rsinfoarea MODIF ID ds. SELECTION-SCREEN END OF BLOCK b2. AT SELECTION-SCREEN OUTPUT. IF all_psas = 'X'. LOOP AT SCREEN. CHECK screen-group1 = 'DS'. screen-active = 0. MODIFY SCREEN. ENDLOOP. ENDIF.
The key is to use the same MODIF ID for all the elements of Block b2.
BR,
Suhas
SELECTION-SCREEN BEGIN OF BLOCK B1 WITH FRAME.
PARAMETERS: ALL_DSOs radiobutton group IP USER-COMMAND flag,
ALL_PSAs radiobutton group IP .
SELECTION-SCREEN END OF BLOCK B1.
SELECTION-SCREEN BEGIN OF BLOCK B2 WITH FRAME.
PARAMETERS: F_DSOs radiobutton group DS modif id ABC,
I_DSOs radiobutton group DS modif id ABC.
SELECTION-SCREEN END OF BLOCK B2.
AT SELECTION-SCREEN output.
LOOP AT SCREEN.
IF ALL_PSAs = 'X' AND screen-group1 = 'ABC'.
screen-active = '0'.
ENDIF.
MODIFY SCREEN.
ENDLOOP.
****Modif ID xxx. is used to group parameters.
