Here's one way to piece batting together. Lay the two batts and overlap them slightly. Use a rotary cutter to ct a wavy line (maybe 3-4 inches wide) down through both batts. Discard the small pieces, and the remaining two large pieces will match exactly. To join them, use a taylor's stitch from side to side. Sharon Schamber demonstrates this type of hand stitch in her basting video:
http://www.youtube.com/watch?v=bhwNylePFAA
Warm and Natural is so stable this technique probably isn't necessary. However, for other battings, and for large quilts, this type of batting join doesn't show even with time and usage.