Unfortunately, our multipleyer testers are fewer than our singleplayer testers, so if you submit as a singleplayer, you are more apt to receive feedback from more than one author. Personally, I prefer that you choose one board or the other to submit your mission to. Feel free to list it as SP,MP and/or COOP when you post it though. If and when reviewed, the staff reviewing it will make mention of it's flexibility.