I use a double needle to hem knitted things fairly often. I sew exclusively on a Singer 403.
Manicmike has good technical information in his post.
Here are some more basic points. You are correctly using two ball point (for knit) needles. When you thread the machine, thread each thread separately, do not pull them through the tension at the same time. If your tension has double tension rings, put the two threads in two different rings. I think your needle is too small. It looks like you are testing the stitch on a cut off t-shirt hem. (Great idea!) If the fabric is similar in weight, try bigger needles, maybe 14? I get skipped stitches when the needle is too small.
Also, see how your fabric is kind of stretched, compared to the commercial hem in your sample? That stretch is happening because of the pull of the presser foot against the feed dogs. What I do on that is cut strips of notebook paper, and "protect" my fabric from the pull of the foot. So your layers are, up from the machine bed, fabric, paper, foot. You just position the paper immediately to the left of where the left needle is entering the fabric. the presser foot just glides along the paper, the feed dogs move the fabric, the fabric slides under the paper.
On a single needle, for knits, I use the single hole soleplate, that helps with the drag, also.