Pepsico said the agreement will double sales of the world's No. 2 soft drink in the Soviet Union by 1990. It expands on an arrangement reached with the Soviets in 1972 under which Pepsico agreed to market Stolichnaya vodka in the United States and the Soviets undertook to import Pepsi concentrate. The concentrate now is used to make Pepsi-Cola at 14 bottling plants. Sales of the soft drink, which costs 35 kopecks (about 40 cents) for a 12-ounce bottle, total about $200 million a year.