Seabourn Cruise Line is expanding on a special promotion it announced last month offering two-for-one international business class airfares to include additional cruises in Europe in length of nine to 15 days. The airfares are available from 38 North American cities for cruises embarking in such cities as Lisbon, Venice, London, Copenhagen and Istanbul. The complimentary airfare has a value of up to $5,800 per couple and may be combined with savings of up to 40 percent off published fares.