Bring sugar and 1 1/2 cups water to a boil in a medium saucepan and cook until the sugar is completely dissolved, about 1 minute. Remove from heat and let cool. Stir in the juices, Cointreau, and tequila. Refrigerate syrup, covered, until cold. For each margarita, pour 2 tablespoons syrup over 1/2 cup crushed ice.