Go ahead: piece your back with the fabrics you have. What you will have when you are finished is really a reversible quilt -- two for the price of one!!
One of my all-time favorite projects is a quilt made of seven dress shirts which I'd shrunk out of. The directions called for using the same fabrics for the back and front -- different patterns, though. I even used the buttons and labels in the thing. It was a lot of fun.