I have done a double layer of batting in several smaller quilts, but nothing as large as a king. I put the batting down and glue basted the backing like normal....then flipped and glue basted the second layer of batting to the first...and finally added the quilt top. After the entire sandwich was dry I put down a few rows of diagonal stitching (temporary basting stitch to be later removed) to secure and prevent any slipping. The only problem I had was bulk....the second layer of batting (cotton, poly or wool) will add a lot of thickness to the quilt and make it more difficult to maneuver. You might want to try a single layer of super thick batting...less issues in the basting stage.