Collaborations on fills are acceptable, as we have several creators in the fandom who usually work together. However, each work can only be used to fill one prompt. For example, you can make a work for "hurt/comfort" and if both cards have that prompt on the board, it'll count towards both cards, but you cannot create a work that fills "hurt/comfort" on one card, and "soulmates" on another card.

Let us know if you have any further questions!