I have a sewing machine with lots of stitch designs to choose from, but 99% of the time I end up just using a plain old straight stitch._ _ _ _
I bought a twin needle a while back, well, just because, but hadn't ever tried it. So in a moment of madness, I pulled it out and gave it a whirl.

I was pretty careful putting that twin needle in. My machine directions say,
"turn your machine off and unplug it before adding your twin needle". Wow. I guess somebody's machine worked when it was turned off and stabbed them with those two razor sharp points. Sounds like I've watched one too many
Lifetime Movies to me.
Anyway, they scared me into being careful. But I'm a rebel and didn't go so far as to unplug my machine.
And yes, my fingers are still all in tact.
Ah, so that's what that extra post is for! If you have two spools of thread the same color, add it to your extra post. If not, you can use a bobbin with the same thread.

It was a little strange threading from two spools down the sewing machine obstacle course...I kept feeling like they were going to start tangling any moment. But they didn't. Both threads stayed where they were supposed to. Just thread each through their own eye.
The twin needle is just what it says it is.
Very parallel.
Perfect finish for a t-shirt or other decorative stitch.
Just watch for corners. No pivot privileges. It's a lift and re-start.