Here's my solution to piecing batting and it has worked great for years. I save all my scraps of fusible make them into strips and use that to piece my cotton and 80/20 batting together. There is a product on the market to use now, but this is leftovers and so no added cost. Just butt the edges of the batting together and apply the fusible.
For polyester batting I just use a very big and loose stitch by hand to hold the edges together.
Longarm