So, how do you use a twin needle? I admit that this is what stymied me for so long. Although I have discovered that Janome is very well suited for twin needle stitching, there isn't a mention of it in her owner's manual (AKA my personal sewing machine Bible). What is up with that?? Fortunately, I found several web and print resources to help me out. I mostly used Sandra Betzina's Power Sewing, which has a nice section on using a twin needle. (Note to self, I ought to buy that book!) There is also a How-To on Sewing.org, and a cool reference chart on Threads. But it really is simple! Here is how. Make sure you have a machine that can zigzag (since that is what the bobbin will be doing). Set your machine for a straight stitch. Put in the twin needle. Thread your machine with two spools of thread with each spool rotating in opposite directions if possible. If your tension discs are visible, thread the separate threads to either side of the discs. Many machines have separate thread guides at the needle holder. If yours doesn't, just leave one of the threads out of that thread guide.
Now sew!
Here is how the stitching will look. The top is the needle stitching and the bottom sample is the bobbin. As you can see, the bobbin makes a zigzag, which leads me to my favorite thing about the double needle. It stretches!
It can also be used for lots of decorative things, besides just plain topstitching. Here you can see what happens when you change up the tension a bit. Set at 0, you can see that the stitches are flat, but once you tighten it up a bit, they begin to mound up just a little in between. By the time we get to an 8, you have a little faux pintuck going on. Cool, huh? You can also use a twin needle with decorative stitching, just be cautious of the stitch width. Be sure that the needles will clear your machine's throat plate before you go to town.
Go to the fabric store! Get a twin needle! Play!!