Well, it appears that my machine doesn't want to do double-needle work. The peg for holding the second spool of thread is broken, so the second spool must sit on the table beside the machine. I think this is messing up the tension as the two threads go through all the parts, hooks, and tension plates.
Or maybe my machine, usually somewhat picky about preferring only Singer needles, doesn't want to play witht the Schmetz universal double-needle I bought.
Either way, it looks like I may be free-handing the double topstitching for any jeans I make. Today I started assembling the muslin/test garment. Good chance to play around with the topstitching and see how homemade the top stitching looks if I do it without a double-needle.
Suprisingly, it's not too terrible! If I do all the topstitiching on the muslin, I'll have gotten a lot of practice before I start my first for-real pair of jeans.
And here was my stopping place as of 8 am this morning (Most mornings I get up at about 6 am and sew for an hour or so). I've decided to not do the topstitching on the right-hand side of the pants, so I can rip that side down to do alterations and pattern adjustments. The top stitching at the yoke on the left side is done with ordinary thread, not my topstitching thread, so as to save the more-expensive top-stitching thread for the final jeans. I also took the precaution of NOT sewing the pockets in place on my muslin, so that I can play around with the pocket placement. (Because pocket placement is key to avoiding the dreaded Mom Jeans Look.)
The yoke construction is pretty neat. You take this piece of fabric that's shaped kind of like half a boomerang, stitch it to the pants back, and BOOM! MAGIC! the rear of the pants now has a curve for your rear end. This is one of the things that I get such a kick out of when sewing: these flat pieces of fabric take on three dimensions when you stitch them together.