How about using Ricky's caveman style cutting. Both fabrics right side up overlapping the edge, and cut your soft curve. Sew together using scant 1/4" seam. You could put registration marks along the way, but if one side is whole cloth there isn't really anything to line up. I made a convergence quilt twin size and the biggest challenge was cutting space. I laid out the fabrics on the floor with the largest cutting matt I could find, when I got the edge of the matt, i carefully slid the fabric over the matt. You could probably pin the fabric that isn't on the matt so it doesn't shift, and remove the pins before you continue to cut. Ricky also suggest drawing your line with water soluble marker if you are uncomfortable cutting a curve for the first time. Hope this makes sense. There is a video of Ricky demonstrating his method, or get his convergence book it really is informative.