Most often the double batt is used on show quilts. The combo I have heard most often is a wool top layer with a cotton or cotton-blend bottom layer. The cotton layer gives stability so the quilt hangs nicely, while the wool 'puffs up' to show off the quilting better.
I would be afraid it would get pretty heavy on a king-sized bed quilt, and it might not drape as nicely either. Perhaps you could try a small wallhanging or lap quilt with the double batt to see if you like the finished result?
Whatever you decide, your quilt is beautiful and will be even more so once you've quilted it!