Years ago someone posted a suggested way to join scraps of batting. Basically, you overlap the two pieces and using your rotary cutter you cut a serpentine line through both pieces. Then, butting the pieces, side by side, and using a wide zig zag stitch you join the two sections. I have found that this avoids lumping and makes the two pieces of batting lay quite flat. So when I need to join batting, this is my go-to method.