Hemming with a twin needle on a home sewing machine.

A twin needle is a specialty needle available in a variety of widths 1.2 - 8mm (mostly 2,3,4 or 6mm) and limited sizes most are 90/14 (11,12,16,18 hard to find almost never in stock. Usually they are universal, if not labeled with a type, will be universal.
Few other types but almost never available, are embroidery, metallic, jeans, stretch.
Today I am using a universal 4.0/90. They tend to be broken often, hitting presser foot or needle plate. I recommend always purchasing several at a time. I buy them from Ebay, no retail labeled, they work just as well as the named brands.
The following is general best practices with a twin needle you machine me need setting a bit different I have 4 sewing machines and all work within in these settings.
They have a single shank (the pat the goes up in the needle holder of your sewing machine, and two needles. You will need two spools of thread, and use the single bobbin a usual.
When using a twin needle you should also lower the needle tension to about 1 ro 2 (1 - 9 scale). Will lessen the fabric between the twin needle from making to large of a ridge or hump. (will do a video on that later)
Also lengthen the stitch a bit, I would recommend no smaller than 3. Will tend to breaks thread with smaller stitch lengths.
You can backtack a twin needle stitch, but don’t recommend that, better if you have to pull needle thread to back and tie off. Reason for no backtacking is that twin needling is usually done a topstitching.
When doing a hem with a twin needle you sew on the right side of fabric and try to get one needle in the folded over part of the ham and the other needle in the single layer of the body of the garment, thus finishing the raw edge of the fabric.