Add a narrow strip of color down the middle of the back. Helps to keep half on one side of the bed, half on the other. (Saw this in a show-and-tell, where the quilter was tired of her dh "hogging" all of the covers. With this quilt, she could show him the real meaning of equality!)
Edit: I agree about getting the mattress cover with dual controls. Much better than making the quilt unequal densities. However, I still recommend the strip of color down the middle of the back!