A 2-needle coverstitch hem is a turned down hem consisting of 2 parallel lines which are 1/4 to 1/8 inch apart on the front side and a binding coverstitch on the back. It is usually found on ready-to-wear clothes, unless you have a coverstitch machine. But it is an expensive purchase and not many have it.
The next best way is to make it with your own sewing machine with a twin needle on it. Not the same, but close.
How to make the coverstitch like hem
There is more than one type of twin needle – depending on the space between the two needles. Choose the appropriate one.
Insert the twin needle (just the way you insert ordinary needle- flat side to the back. If in doubt, consult the service manual of your sewing machine).
Wind the bobbin without much tension.
Thread the sewing machine. You will need two thread spools and two places to keep both of them on the machine. I kept one on the bobbin winder and the other on the usual thread space. Thread the machine as you do, both of the threads together through all the usual places.
Then when you reach the needle eyes, thread the needles seperately with the two threads.
You may need to check the service manual for your sewing machine for steps for your particular machine. General steps are as given below.
Bring bobbin thread up as usual. Hold it all together with your hands.Take the two threads and the bobbin thread to the back of the machine under the pressure foot.
Set the stitch for sewing the straight stitch.
Adjust the stitch length to 2.5 to 3 mm.
Twin needle used along with a longer stitch length will make a hem similar to a coverstitch hem.
Loosen the top thread tension a little bit.
You will be sewing from the top on the hem line. After folding the fabric to the back twice, just sew from the top with the longer stitch length and twin needles on.
Some thin fabric may need stabilizers / backing added to the back of the fabric to prevent thread pulling in the fabric between the stitching lines. This is especially true for knits.
You will have to hold the fabric taut and may also need to use a stretch double needle when sewing knit for best results.
Remember to keep the thread tails to the back of the needle at all times during sewing.
You will get a coverstitch like stitch on the back.
Checkout the post on twin needle stitching for more details.
A concern about the hem
What to do about the tunnelling between the two stitching lines?
This hem is best on a woven fabric because twin needles essentially works best with woven fabrics. The tunnelling is a common problem with this types of stitching. It is very minor but if you hate it, you are going to hate it. You can apply fusible tape to stabilise the area underneath. When sewing a stretchy fabric with this kind of hem, slightly stretch the material as you sew to prevent the tunnelling in between the stitches.