When I piece batting, I follow a tip I read somewhere and lay the two pieces slightly overlapping on my cutting mat. Then I use the rotary cutter to make a serpentine line cut through both layers. Then I but the two matching serpentine edges together and zig-zag. Supposedly, this helps the quilt not to develop odd creases and folds in it later due to the pieced batting, and allows more of the seam to be caught under the quilting, strengthening it.