I save nearly all my batting pieces that are leftover from a quilt. You never know when you might need a small piece for a mug rug, a wall hanging, a small bag, or even an ornament. I had some big pieces leftover from when I had to buy a larger size batting because the shop was out of the size I needed. So to use these leftover pieces, I take my two pieces of batting and lay them side by side.
In order to piece them together, overlap the batting edges just enough to give you room to cut. Usually just an inch or so will do, depending on how straight the edge is.
Next, cut through the layers with your rotary cutter. The good part is this doesn't have to be straight! In fact, a gentle curve is best.
After you have cut through the layers you will be left with two narrow pieces of batting. I toss these, but they would make good stuffing for doggie beds.
Butt the edges of your batting pieces, no overlap, cut edge to cut edge.
Sew with a wide zigzag stitch. Or, there is now a fusible web tape that you can iron on the batting to join the pieces together.