I recently discovered the wonder of a twin needle. It's actually quite simple to use, and sews perfectly parallel lines. The most difficult part is threading the needle; I recommend reading your owner's manual. My machine has a setting specially for the twin needle to prevent the needle hitting the presser foot (and breaking). If your machine doesn't have a different setting, be sure to sew in a straight line, with the needle centered.
My machine has two different tension plates, so the two top threads each take a side. I tried taking multiple pictures, but none came out great.
I used three different colored threads to better demonstrate exactly how the twin needle works. Two of the threads are threaded through the top of the machine, and each threads goes through one side of the needle. The last thread is the bobbin thread. I used white for my main thread, brown for my secondary top thread, and red for the bobbin.
Did you know that you can buy twin needles in different widths? Each width gives you a different look, but functions the same way.
You can sew around curves with the twin needle (like I did on the reversible bag), but be sure to sew very slowly. If you sew a curve too fast, you will place pressure on the needle, and it will break.
Now go and get a twin needle, so you can sew perfectly parallel lines!