Double needles sew 2 lines of straight stitches on top...underneath the stitch looks similar to a zig zag because there is only one bobbin thread to lock the two top threads together. It is used mostly for top stitching hems for a finished look where the back side doesn't show. I would remove the needle plate, install a double needle, and turn the wheel by hand to be sure the needles pick up the bobbin thread. Then make a decision re the new needle plate.