You can piece batting. What type of batting is it? With cotton, you can do a long machine zigzag to join two pieces of batting. Or, you can use batting tape and iron it on (or make your own batting tape from fusible). With polyester I would hesitate to use the iron-on option and, if it's high loft, I might not like the machine zigzag. In that case, I would do a hand stitch to join -- probably a herringbone similar to the stitch Sharon Schambers uses when basting quilts with her boards (Youtube video demonstrates the stitch). Or, any kind of hand stitching that keeps the joined batting pieces together!