I am stitching over one on 28ct fabric. Although I have not done the backstitched arch yet (I will probably do that last), I plan to use short diagonal backstitches in the area between threads 18 and 30, going from point to point, and long stitches between threads 30 and 80. The same on the opposite side. The horizontal and vertical stitches shouldn't be a problem. I'm not sure how smooth of an arc we're going to get, but I don't see any other way to stitch this arch, regardless of whether you are doing it over one or over two.