Hi Faisal,
i think value which is you have passed that will vanish as u said..
in first drop down ( dorp down A) create event in that capture the selected dorp down value into static variable and then go to (drop down B) get_p method check the value based on that set the property as mandatory.
same way get the dropdown value in do_prepare_output to because when ever your click on any other fields it going to round trip so the earlier selected value is vanished so that again we have to capture the drop down( A field ) value which is already selected.
Thanks & Regards,
Srinivas.