Is the back quilt & the front quilt both exactly the same size? I have never done this, but wonder if it is .... you could lay your batting down and either spray baste or pin baste your backing to it, and then extend some pen or thread markings out into the extra batting. Then flip the whole thing over so the backing quilt is on the floor with your batting side up and you can see the markings you made out into the extra batting. Lay your top quilt down lining it up with these markings. Does this make sense?