I've been asked a couple times in IRC if collaboration is allowed.
Working with a partner is technically allowed, but please do not indicate the collaboration when submitting. It will be treated as one person's entry and only the submitter will be rewarded a prize. Also remember that you are judged on the cohesiveness of your beatmap set, so it may actually be disadvantageous to work with another person, especially if two people are making unrelated difficulties for the same set. If you do collab, you should be working very closely together to make sure everything feels consistent... Ideally, the judge shouldn't notice that it's not the same mapper.
So in other words, you can do it, but I don't recommend it.