These "screw-apart" cold cups are hard to find. They just don't make them like this anymore. If you see the cold cups for sale in Starbucks, they won't screw apart into two cups. So, if you do find some, hoard them all and sell them on eBay for $100 each!
I know that there are many tutorials out there on how to get your double-cup all glittery, but here's mine. So simple!
Double-cupped Starbucks (or any drink cup that unscrews)
glitter (the bigger the better, I used fine glitter, but I think I would have gotten a better effect if I'd used larger glitter)
Tape off the inside top rim of the outer cup (not the inner cup), so that the glitter doesn't get into the grooves of the screw-on cup rims.
Spray the glue into the interior of the outer cup. Don't get it on the outside of the cup, or you will have problems. "I got 99 problems and this ain't one!"
Then apply your glitter down the sides of the cup. I tried to be tricky and do it in stripes, but you can't really tell. If you use more than one color glitter, use the one you want to show the most first!
Let dry, take off the tape:
Have an awesomely glitter-fied day!