I haven't yet, but I'm about to do a double-batt king size as soon as I get done with a baby quilt and a wedding quilt I'm on a deadline for. So I'm watching this thread with interest!
I'm going to use a layer of cotton and a layer of wool. I know it'll be heavy - that's what we want; this is to be our winter bed quilt and DH in particular loves a heavy quilt. My idea is to spray baste between the layers of batting first so I end up with one very thick batting to work with. Then sandwich & quilt as usual. I hope it works! The top is done - I made it with large 12" blocks of fabric so I don't have a lot of seams to fight with, hopefully that will help! And I intend to keep the quilting pretty spare so it will have maximum fluff; both bats have 8" max; I will probably quilt at about 6" or so, probably in a simple on-point grid.