Process half of the lemonade, limeade, and ice in a blender until smooth, stopping to scrape down sides. Pour mixture into a 4-quart plastic container. Repeat procedure with other half. Freeze until firm.
Remove from freezer 30 minutes before serving; break into chunks. Add soda; stir until slushy. Garnish, if desired.
Note: In testing, we used 7-Up for the lemon-lime soda.