I originally thought I'd sew up two widths of ribbon, laying one over the other, on the sides to make the full 2 inches of the pattern. Like this.
But that's not pretty. Then I realized if I used one 1 1/2 inch ribbon width at the sides, instead of combining two to make it wider, and used the full 1 1/2 inches at the front busk and back lacing, I'd gain some of that width back. In fact, the finished corset will be a bit wider than the pattern because the front and back panels aren't 1 1/2 inches wide, but that's OK. A ribbon corset isn't design to cinch several inches off the waist, so an extra half inch won't make a difference.
I laid the two halves together, face-to-face, basted the top edge of the side ribbons together, and flipped so the right side is out.
I could continue sewing by hand, but I've arrived home and my sewing machine is calling me. I hope to knock this ribbon corset out over the Thanksgiving weekend so I can get back to work on the 1925 corselet I left behind. It hasn't been forgotten!