The process for gathering using a double row of stitching is essentially the same as the single row method. As before, use a longer stitch length and sew without backstitching at the ends. The difference here is to sew another row of stitches parallel to the first row. I like to sew one row inside the seam allowance and the second row on the other side of the seam allowance. I then sew the permanent seam in the middle of the two gathering rows. Having a gathering row on both sides of the final seam helps stabilize the gathers while you are sewing the seam.
To gather the double row of stitching, hold both bobbin threads on one side while sliding the fabric along. You want to gather them at the same time - if you gather the fabric using one thread at a time, the gathers can end up leaning diagonally instead of being straight. Adjust the gathers until the fabric is the desired length, making sure the gathers are evenly distributed along the length.
The double row method creates gathers that more even and easier to control than those made with a single row of stitching, so this method can be used for most types of applications and fabrics.
When gathering long pieces of fabric, it is helpful to divide them into smaller sections and sew them individually. An easy way to do this is to stop stitching at the end of one section with the needle up and raise the presser foot. Pull the fabric about six inches away from the sewing plate and then resume sewing again just past where you left off. Make sure the loose threads do not become caught in the stitching. After you are finished stitching, you can snip the bobbin threads when you are ready to start gathering the fabric.