Just a few tips to make sure you get this and don't disqualify you or your friends from this achievement
-One player must have beaten the entire Co-op before and one player must have never done the tutorial.
-The player who has never done the tutorial must never been in a chamber, the central hub is fine.
-Single player does not affect this, so you can have played the whole single player and it won't affect this.
-If you are going for other achievements, such as Friends list with benefits or other similiar co-op ones, skip the tutorial by pausing during the opening cutscene and returning to the hub.
-This cannot be done split-screen, must be two players with Active X-box Live accounts
-The only required chamber is the tutorial chamber where you acquire the gun and calibrate weight variances, which can really upset humans if you bring up :)
Hope this helps.