OK, can you tell I'm on a roll, with all these posts? I wanted to ask you another question, so will put this as question of the day. Are you familiar with twin needle sewing? Have you ever used it when quilting?
I found a double needle in the attachment box of my new machine and it intrigued me, so I went searching for information on how to sew with one. Today, I broke it out and used it for the first time when working on my baby quilt, instead of trying to make a narrow double row, one seam at a time. I birthed this quilt and wanted to run a twin seam about 3/4 inch from the edge all around the piece, as sort of a faux binding, but without the added weight and stiffness.
I am just impressed because the seam is automatically perfectly straight. Any machine can use a double needle and they come in different widths, so you can choose how wide you want to make your double seam. The back of the work leaves a very pleasant pattern as well, sort of like a zig-zag decorative. Both needles catch up the single bobbin thread at once.
I know how difficult stitch in the ditch is, and know it's often recommended to go slightly to one side of the seam to keep the line accurate. My thought is that this could be a good solution....to straddle the seam with the twin needle, sort of encasing it between the rows. I could see where the encasement would be an added benefit for durability in quilts one would expect to launder frequently, like a baby quilt, adding strength to the existing seam holding the pieces together, sort of like the intention of sashiko.
Here is a link that might be useful: using twin needles