The easiest solution to keep curtains together in the closed position in the middle of the window is to apply adhesive Velcro (hook and loop) tape. Attach a few pieces of the Velcro along the edges of the curtains where the two panels meet. Place a piece on the near the bottom, middle and top to ensure easy closure without a gap along the entire length of the curtains. Depending on the fabric and pattern on the curtains, you can place the Velcro on the wrong side of the curtain so you don't see it when the curtains are open. Velcro tape is available both in sew-on or adhesive varieties as well as pre-cut strips and circle shapes in both black and white.