Originally Posted by wolfkitty
Are they to be used for metallic thread, with the second thread to strengthen the metallic thread?
The double eye sewing machine needle I cited is made for metallic thread, but would work with any kind of thread. For that one, you would use two different colors of metallic thread, or two different colors of any kind of thread, or two of the same color if you wanted, I suppose. Metallic thread does not need to be strengthened.
Any regular double eye sewing machine needle is meant for a decorative stitch, and meant to use two different colors of thread so they will both show up in the same stitching.