Cat - N
I cannot answer the question about using 4-threads (including 2 needle threads) to create a flatlock stitch on a serger other than to say that the 1200D manual shows a 2-thread flatlock and a 3-thread flatlock stitch. My Pfaff serger does not have a flatlock stitch that uses 2 needle threads either.
The flatlock stitch, being an "unbalanced" stitch, uses, in the 1200D, 1 needle and only the lower looper to create the 2-thread stitch, and uses 1 needle plus both upper and lower loopers to create the 3-thread unbalanced flatlock stitch that can be pulled flat after stitching like you see on some ready-to-wear garments.
I am a bit out of it on this "serger issues" subject, having injured my ankle on Tuesday, and now hobbling around in a rigid, inflated boot, getting a whole bunch of nothing accomplished quickly, so when I finally got back to the PC for a little while, was hoping to find an email saying that June was able to get her 1200D to sew the flatlock stitch on organza that she needs.
In case I do not find such an email, I am going to attempt to attach pix of the 2-thread and 3-thread flatlock stitches from my Pfaff manual with instructions for tensions adjustments, and in PDF format, too. Each thread is represented as a different color, making it easy to see what is being described...maybe the attachments will come through and help in some small way.
- Cat (FL)