John Flynn has a free lesson on his website on how to figure and piece your backing on the diagonal. You could piece your batting layer and backing layer in opposite directions so there's not a seam in the same place in both layers. The only place there would be a double layer is where the seams cross. Here's the link:
http://www.flynnquilt.com/media/uplo...onalpb1103.pdf
If that doesn't work, just go to the John Flynn website and click on "John's Workshop" to get to "Free Patterns"