The only 2d editing available at the moment is to modify the position of the reference line, and to add additional points to it. The position of the mullions still can only be adjusted in the 3d window I'm afraid. It's a pain, and hopefully something they will improve in future releases because the floorplan is the obvious place where you would want to edit the mullion positions!
A workaround could be to place temporary walls at the points where you want the mullions to go, and then use those to snap to when editing in 3d.
