Hi Naveen,
1. To track the chechbox : When the user selects any of the checkboxes , internally the field will have a 'X' value i.e ATAB-aprove or ATAB-reject will hold a X value.So, this value will help you to track the field whether it is checked or not.
2. To save the particular line : Check for both the fields with 'X' Value.
3. To give a pop-up : If both the fields have a value 'X' then, use a Function module POPUP_TO_CONFIRM to raise a pop up.
Hope this helps. Do let me know if anything else is required.
Thanks,
Namrata