Thank you so much to Mamafitz, Gwen, Nicole, Alison, Lisa Laree, Julia, Carla and Patsijean for talking back to my confession last week! I asked about twin needles - something I've never used on my sewing machine.
People reported three uses for twin needles.
The least commonly mentioned use for twin-needles was top stitching (double row, obviously). Only 2 people called this out.
The second use of a twin needle mentioned by the commenters was for pintucks - and people were kind of divided on this one. Four people reported having tried this technique, while three people have never tried it - and not everyone likes the effect. Mamafitz, for example, prefers the look of traditional pintucks.
A couple of people mentioned that their machines provide specific pintuck feet to be used with the twin needle. Gwen's Berina, for example, actually has 4 different feet - each one designed to make pintucks of a different width.
She also passed along a tip - she pintucks a block of fabric first and only then lays down her pattern piece and cuts it out. This allows her to get the pintucks precisely where she wants them on the garment. What a great idea!
Alison provided a very specific recommendation regarding needle size - 1.7/70 or 2.0/80. And Mamafitz recommended the book pictured above, Fine Machine Sewing by Carol Ahles, to learn more about using special machine needles like twin needles and wing needles. (Yikes! I've never even heard of a wing needle!)
Finally (saving the best for last) the most commonly called out use for a twin needle was hemming - with 6 people emphasizing knits and 1 additional person saying that really they can be used to hem lots of different fabrics.
Generally people seemed to like this approach to dealing with hems on knits, although Carla has had issues with tunneling (even with special thread) and Patsijean switched to using a cover stitch machine instead, once she had that option.
As usual, I got lots of good advice - considering hemming knits in particular, Alison recommended using twin needles designed for stretch fabrics and the size 4.0/75. (Apparently the twin needle in my photo - 6.0/100 - is a bit on the large side!) Lisa Laree recommended using wooly nylon bobbin thread for this, as it has more stretch and thus doesn't break as often. And Patsijean noted that you may have to loosen the top tension a bit and test your stitch on scrap fabric before sewing the real thing.
Thanks again to everyone who took the time to leave a comment!