Create a new 128x32 sector (S7) off the south edge of S1, and turn it into a door in the middle as in steps 7 and 8, but this time setting the action as Door (Blue) Open Stay. Add a larger area (S8) at S7's south end. Click V to enter Vertices mode. Set the grid size to 32 with [ or ], then on the bottom edge of S8, right click one square to the left and right of its center. This splits the linedef/edge into three parts.