I quilt with two batts quite often. My preferred method is a layer of low loft on the bottom like W&N, 80/20, bamboo, I have even used an old wool army blanket. Then I usually prefer a higher loft on top like Hobbs Poly down. I do this to get definition in the quilting. Each layer I use the same batting but I have used pieces. The low loft batting I zig zag stitch together but the high loft I have simply butted one up to the other.