Hi Lois - I haven't done it, but I've seen information on how to do it. I believe it was an old "Simply Quilts" episode where Linda Milligan and Nancy Smith talked about this and they suggested that you lay the two pieces of batting next to each other, right sides up, overlapping a bit. Then take a rotary cutter and cut through both battings, using a cut that curves back and forth. Then butt up the edges of the batting and sew together with a wide zigzag stitch. The curvy cut was important because it kept the batting from shifting and it made sure that you didn't have a noticeable line on the quilt where the battings butted together.
There's also a new product that is a fusible for connecting batting pieces. Again, I haven't tried this myself.
Hope this helps,
Nancy