Stir together 1 cup of the grapefruit juice and sugar, if desired, until sugar is dissolved. Pour into an ice tray and freeze into solid cubes. In another ice tray, pour 1 cup of the pineapple juice and freeze into solid cubes. The fruit mixtures may be prepared up to this point 1 day in advance.
In a blender, blend frozen grapefruit juice mixture with 1 1/2 ounces of tequila and the remaining 1/4 cup grapefruit juice, scraping down the sides occasionally, until the mixture is smooth but still frozen and divide the mixture between 2 stemmed glasses. In the cleaned blender, blend the frozen pineapple juice with the remaining 1 1/2 ounces tequila and the remaining 1/2 cup pineapple juice, scraping down the sides occasionally, until the mixture is smooth but still frozen and divide the mixture between the 2 glasses.
