I whip stitch the two together by hand. It goes rather quickly. I saw a tip however to use a strip of thin muslin and sew it to one piece of of the batting with the one edge of the muslin projecting beyond the batting. Butt the other piece of batting under the muslin and then sew that edge. Supposedly you don't end up with the bump that you sometimes do when you zig zag the two pieces of batting together. I have never tried this method but perhaps others have and can comment on it. You can also buy tape for the joining but it is too expensive for my taste.