When I read "double Halloween costume" I thought it was gonna be one of those things where your top half is Igor riding on Frankenstein's back, and your bottom half is secretly Frankenstein's legs, or that similar ostrich jockey one, and they make it look like your body is riding on some kind of creature, but really you're both of them.
For those about to say "HEY FRANKENSTEIN'S NOT THE NAME OF THE MONSTER", the monster took Dr. Frankenstein's last name in many adaptations and semi-sequels (glorified fanfics). So there. Kind of. Somewhere around there in that general area.