I overlap the batting, then cut a wavy line within the overlapped section. Make sure the peaks and valleys of the wavy line are not extreme so that it is easier to manipulate through the machine as you sew. Separate the two pieces of batting again and remove the wavy scraps. Then abut the two pieces of batting, interlocking the wavy lines. Zig-zag to join the pieces. I find this holds very well and the wavy seam is practically undetectable below the quilt top vs. a zig-zagged straight-line.