The reason for the surged look on the back is you have only one bobbin, and both needle threads have to interact with the one bobbin thread. There is no way you won't get that sort of thread cross over look on the back. There is no way possible to sew with one thread going through both needles. The reason for twin needles is so that you see two lines of stitching on the front. I did some top-stitching with a double needle with a zit-zag stitch, and it was beautiful. I used Number 22 on the Bernina. You can't just sew with the width that is normal for that stitch. You have to put your needle down with your hand on the fly wheel until you get the width just right, or the needles will be too wide for the space in the throat plate. If you try that, be real careful.