Heat 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.
In 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.
In medium bowl, mix both containers frosting and remaining package drink mix.
Spoon frosting mixture into heavy-duty freezer bag.
Cut corner off freezer bag.
To 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.