I do this frequently but I use the iron-on tape. I works so well and I have never had any problems with it. A small investment that always works is good for me. I always butt up straight edge to straight edge and iron carefully following the directions on the package. Easy, peasy and always successful.

I zig zag them together all the time. To get the edges the same I overlap them a bit and then cut through the overlap. Take away the thin strips on both pieces and your edges will match perfectly. I will often have more than one seam in the batting in some quilts. The small pieces of batting get used for pot holders and quilt as you go squares and even then I've had seams in them. I do make sure that it is the same kind of batting. I use mostly Warm and Natural but I do also have some Hobbs 80/20. I don't mix the two.

Thanks everyone for you responses. I'll give it a go!

Me, too. Have had great results. I use W&N, almost always. If I thought I was making an heirloom or a work of art to be judged in a show, I might hesitate to piece batting by any method, but that's not me. One word of caution, though. I have seen several antique quilts where the batting was torn along the edge from people tugging and pulling the quilt up under their chins. If you have to piece batting, it would be best to place the joined area where it won't get that additional stress.

My machine has a stitch that puts a straight stitch after each zig- or zag-. I like the added stability this gives to pieced batting. I have not had any problems piecing W&N scraps without considering which direction they may have originally laid.

I have used different methods of piecing the batting...zigzaging the edges together, using the special iron on tape or cutting a wavy seam through both layers and removing the excess before joining the edges together. Another method, while it does take a bit more time, is flat locking the edges on the serger. This gives a nice finish. I do plenty of quilting so that the edges will not come apart no matter which method I use.

This is something I've wondered about too. Thanks for all the great advice!

I do this quite often with good results. The only thing I do differently is after cutting the straight seam I add some pellon fusible interfacing. I see they now sell it as a roll but I usually just buy it by the yard and then using a cloth (because I usee polydown batting) iron it in place. Then I go over it with a zig zag stitch. Sorry I can't remember what the name of the pellon interfacing is, it is very sheer and looks like a very light weight netting.

I join 2 pieces all the time but I tear the edges so there isn't a cut, sharp edge. I over lap the torn edges and don't stitch them together at all until the quilt is quilted. I just lay them out with the top and back. When inside the quilt sandwich they stay together just fine. There is no ridge and it isn't at all noticeable on the completed quilt.

That is my method too, and it really helps.