If you use a twin needle to do topstitching, be aware that the stitching on the underside is very different than what you might expect; it form a zigzag between the two rows on top. This makes it nice for topstitching knits, because the zigzag underneath gives the stitch some stretch. The bobbin tension usually needs to be loosened for this type of stitch; otherwise the fabric creates a "tunnel" between the two lines of stitching (from the zigzagging being too tight).