what was the drug which in 08/2102 had been prescribed to patient 6440 two times?
select t1.drug from ( select prescriptions.drug, count( prescriptions.startdate ) as c1 from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 6440 ) and strftime('%y-%m',prescriptions.startdate) = '2102-08' group by prescriptions.drug ) as t1 where t1.c1 = 2