1Heat oven to 350°F (325°F for dark or nonstick pans). Place paper baking cup in each of 24 regular-size muffin cups. Make and bake cake mix as directed on box for 24 cupcakes. Cool in pans 10 minutes; remove from pans to cooling rack. Cool completely, about 30 minutes.
2In 1-quart saucepan, mix 1 cup water, sugar and 1 package drink mix. Heat to boiling over medium-high heat, stirring until sugar is dissolved. Remove from heat; set aside to cool.
3In medium bowl, mix both containers frosting and remaining package drink mix. Spoon frosting mixture into heavy-duty freezer bag. Cut corner off freezer bag.
4To assemble each of 12 slushee cups, remove cupcake liners from 2 cupcakes. Put 1 cupcake in bottom of drinking cup, and poke with fork. Spoon 1 tablespoon drink mix syrup over cupcake. Top with about 1 tablespoon frosting mixture. Put second cupcake on top of frosting in drinking cup, and poke with fork. Spoon 1 tablespoon drink mix syrup over cupcake. Pipe frosting over top of cupcake. Insert straw into cup. Sprinkle with edible glitter. Repeat to use remaining cupcakes. Refrigerate at least 1 hour before serving. Store any leftover cupcakes in refrigerator.