This must be done in Perform It mode, where co-op is allowed. This can also be done on any difficulty. This obviously requires two people (don't try to use an inanimate object) since you'll both be dancing simultaneously. Assuming you're both playing on the same difficulty, you'll both be dancing the same moves at the same time throughout the song, along with the standard freestyle section. Find a partner who is adept at dancing or let your partner follow your lead. Don't think you can carry your partner if they don't know what they're doing. It'll only get you so far.