Take down and disassemble the Day-Night shade. Use your screwdriver to remove the screws from each end of the upper shade, or the night shade, and detach the shade from the clip in the middle. Set the Day-Night shade on the floor and pry the end caps off the rails with the screwdriver and slide out the metal bars. Remove the spools from the cord and cut the cord from the spring at the top of the shade.
Thread the night shade. Cut your 20 yard cord in into two 10 yard sections and fold them in half as you loop them over the opposite ends of the spring--one on each side. Duct tape the spring to the center of the top rail of the shade. Center the cords on each side of the spring and slide the top bar of the night shade back in without catching the cords as you do it. Put the ends of the cords on the left and right sides through the eyes of your needles and thread the cords through the holes in the upper shade, the night shade.
Attach the day shade to the night shade. Cross the left cords over to the right holes and the right cords over to the left holes in the top of the day shade and thread them through a few holes. Slide the top bar of the day shade back in without catching it on the cords. Reinstall the end caps to hold the two bars in place and pull the ends of the cords until the night shade closes. Thread the cords through the remaining holes in the day shade until you reach the bottom. At the bottom, cross the cords over to the opposite side and slide the bottom bar of the day shade back in without catching it on the cords. Feed the cords through the holes in the end caps and push the end caps back in to hold the bar in place. Pull the cords tight to close the day shade.
Reinstall the shade. Insert the center top rail of the night shade into the retainer clip above the window and put the screws back in at each end. Leave enough cord on each side of the window to allow for three wraps around the spools. Cut off the extra cord and reattach the cords on each side to their corresponding spool by threading the cords through the holes in the spool and tying knots in the ends to keep them from coming out. Adjust the spools with your screwdriver until the cords are tight enough to let the shades move up and down smoothly.