rather than quilting two layers, you could go old school and use the shingle method. Cut strips about a foot wide (width isn't important, but keep them all the same) Sew a strip on the TOP edge along the bottom, overlap the next by 1/2 and sew the top again, and continue all the way up. Now go back and hand sew your top sheet on JUST catching the insulation with the needle -do not sew through the insulation, you're just keeping the top sheet in place. You now have 2 layers with minimal compression and no cold spots.
Another way is to sew the insulation the same way to start, but don't overlap. sew a second sheet identical, lay them top to top, and mesh them together- Flip one layer up, pull over and flip the other under it, and continue until they're all meshed together and sew the edges- less hand work, but you're relying on the friction of the insulation to keep it all in place.