I like to join pieces of wadding (or quilt batting) together – it’s such a waste throwing out the off-cuts.
I abut the two pieces together – no overlapping and join them with a zig zag stitch. I use a multiple zig zag, but I’m sure a normal zig zag would work as well.
I’ve used this method several times and had no problems with the wadding coming apart or shifting. You just want to make sure there isn’t any lumps along the join (no overlapping)