Co-op forges never work out well because of the fact that you need to restart round for fusion and save and quit for hovering... One player will always be irritated. Unless you mean a chain sort of thin, where you pass over the files and let another player add to it, which can be equally as disastrous.
ya, co-op forging isnt the best idea, but don't get me wrong 2 heads can be better than one. what i do i i get a friend and then we think of ideas, we pick the best one, and we make our own separate maps and then compare and get more ideas.