I lay a strip of iron-on batting tape, sticky side up, on the sewing machine, and butt 2 edges batting pieces in top. I zig zag a wide stitch catching the two pieces of batting with the tape underneath. When I am done stitching, I take it to the ironing board and press the sewed on tape to the batting for reinforcement. This takes out the bunchiness and make the pieces act like one batting.
this is the tape:
http://www.joann.com/batting-tape-3q...2Btape&start=2