Combine the grapefruit juice, tequila, lime juice, and simple syrup in a quart container and freeze it for at least 6 hours (or overnight), until solid. Break up the mixture with a fork and transfer half of it to the jar of a blender. Add 1 cup of the ice and 1/8 teaspoon salt and blend until smooth. Divide the frozen mixture between two highball glasses, then repeat with the remaining mixture, ice, and salt. Garnish each glass with a grapefruit wedge and serve frozen.