The other officers in the line of succession do not become president in those cases. Instead, the next available person in line merely becomes acting president, and the Constitution dictates that, whenever both the presidency and the vice-presidency become vacant, new elections are to be held. So in the case of an acting president taking office due to a double vacancy in the presidency and vice-presidency, the acting president serves only until the special election takes place and the newly elected president and vice-president are sworn-in.