What does Double Needle mean?
A stitching method in which two threads are sewn side by side for strength, which is commonly used in denim.
Heddels explains Double Needle
You guessed it: a double needle is exactly what it implies. Also called a twin needle, it is used to create two perfectly parallel seams. Using a double needle gives jeans a more finished look while also strengthening the seams.
Here is an example of double needling: