Prism99

You get 2 parallel lines on top, but the backside is a zigzag. In order to prevent tunneling, you need to reduce upper tension a lot. Test for yourself, but I did not like it for quilting.

For red work, are you wanting a heavier line? If so, I would just double the thread (use 2 spools and treat the 2 threads as one when threading the machine) or use a heavier thread. A double needle could be nice for straight lines, but you can't really turn corners with it. It also is probably not very good on curves unless the curves are quite large.

You typically use a double needle only for applications where you don't care about the back -- for example, a wallhanging, or the hem on a child's dress.