Either of those options is permitted; since there's no possibility that a Madness treat could be turned into a recip's main gift, there's a much more laissez-faire attitude towards the collection in general.

That said, most people try to give treats that match the prompt as closely as possible. I've seen Madness stories gifted to multiple recips when those recips' prompts were very similar and there was no real way for the author to choose among them, but I don't think I've ever seen a story gifted to multiple recips just on the basis of its fandom.