When ekadashi (the eleventh lunar day of the bright half of the month) does not fall on two days, Dvadashi (12th tithi) should be observed on the next day. But when it falls on two days, ascetics and widows should observe it on the second day. A dual tithi does not exist on the second day. Still it is called such. Only the first day wil dominate. If there is an unvitiated 12th tithi, that alone is the best.