You can sew two pairs of equal-length curtains together to make one pair. You can use either two pairs of matching curtains, or curtains in complementary colours. To get the best result, you will need to use both a zigzag sewing machine and a hand sewing needle. Line up the casings for the curtain rods before you begin sewing, and confirm that the curtains are the same length. The curtain hems will line up after you align the casings if the curtains are the same length.
Place the curtain panels side by side, overlapping them 1/8 inch, and fasten the casings together with the small safety pins. Place the pins near the top of the casing so the sewing machine needle will not catch them.
Set the sewing machine on a wide, medium length zigzag stitch.
Beginning just under the bottom casing seam, place the overlapped edges of the curtain panels under the sewing machine presser foot. Center the overlap under the needle, so the zigzags will cover the entire overlap when you sew. Sew to the end of the panels, stopping every six inches to make sure the 1/8-inch overlap remains straight.
Insert the curtain rod into the curtain casing. Pull an 18-inch piece of thread off of the sewing machine, and thread the hand-sewing needle.
Beginning on the back of the curtains, hand sew the casings together. Make short, tight slip stitches that go into the fabric of one panel and come out on the other panel. Slip stitch all of the way around the casing to finish joining the panels.
Before sewing the panels together, stop and take a moment to be certain the panels are placed the way you want them. Putting the curtain rod into the casing will prevent sewing the casings closed, but it makes it awkward to hold the panels while sewing. A zigzag stitch is not normally used to fuse a seam, but the zigzag stitch works for joining curtain panels because curtains are not exposed to the same amount of stress as clothing or upholstery. When selecting a hand-sewing needle for this project, look for a needle labelled as either "Betweens" or "Quilting."