I put them edges together, and use the widest longest zigzag stitch.
Holds well without bulk and without flattening.
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First, I'm aware this is an old thread.
Second, I found it by trying to figure out how to successfully join polyester batting pieces. I did not find an answer that really worked well. So I tried numerous approaches and this is what I discovered: the best, most secure way of hand stitching polyester batting together is the buttonhole stitch. Kept it secure, didn't pull apart, and was quick. Thanks for listening. I hope this helps someone else too.