Like others here, I just do a big running whip stitch

I'd drag out some buttonhole twist thread and whip the pieces together. I've done so more than once. Put your needle into the batting at least a half an inch and don't pull up too tight, you don't want the batting overlapping.

I use a blind hem stitch, but I don't butt the 2 pieces together. I stack them, cut a nice straight edge, pin and stitch with the straight part of the stitch to the right and the zigzag part to the left. Then I open it up and rub my finger along the seam to smooth it out. I love how it comes out and you can not tell where the seam is when it's done. I find it easier to handle this way than to butt them together - especially if the pieces are large.

I used to put them side by side with an overlap, cut a wavy line and hand stitch the pieces together. It comes out nice that way, and you would never know it was pieced, but can be awkward to handle. I prefer the blind hem stitch way best. Try a few ways on some scraps and see what you prefer!

Another reason I like slip stitching is because it doesn't cause the batting to be flattened at the new joint.

I just used the fusible strip last night to piece two pieced of batting together and I love it. I ended up with a virtual seamless piece of batting. I ordered mine on-line, can't remember from whom, but I remember it came in varying widths. I bought a roll 1/2 inch wide. You simply place your batting pieced together, lay the tape over the seam and press on a wool setting. Almost instantly I had one large piece of batting. Works wonderfully!

I use the iron-on heat tape as well that comes on a roll but cut it in half to 1 inch strips and it works just as well and the tape goes twice as far.

This is what I do. I have a very lightweight fusible that is 45" wide. I cut it in 2" or 3" strips and then fuse the batting together. Then I use my largest stepped zig zag to go down the center to help reinforce it. Works great, and I have been getting rid of lots of batting scraps.

Just use a triple step zig zag or a joining stitch. Works great.

I hand stitch mine together with a large stitch, butting the edges together. I've found that if I zigzag it, or sew it on the machine in any way, I have a ditch where the seam is. I just don't like the way that looks on my finished quilt.

I have used the tape & it works wonderfully well but most of the time, especially on long, pieces, I just zigzag & it works just as well. Just make sure you butt the pieces right next to each other as close as possible.