I make mostly art quilts and use a longarm. Art quilters often use two layers, with W&N on the bottom and wool on top (next to the quilt top) because of the definition it can give to the quilting. Some art quilters use a layer of felt in addition, next to the backing because it helps the quilt hang straight. Others interface the backing for the same reason. I find that even one layer of W&N in a bed size quilt makes the quilt too heavy for me.