If I have to piece together batting, the first thing I do is overlap the two pieces that need to be put together and use my rotary cutter to cut a stright line through the overlapped section. That way, it fits perfectly.... no bulges or gaps when you stitch. I throw away the tiny strips that are left over and hand stitch the two together..(big X).. I don't like to move them because they might stretch.
I try to seam the backs so that the center section is a full width of fabric (40-42"). Then I slice the 2nd piece of fabric lengthwise and stitch to both sides of the full width in the center.
I hope this makes sense.