I usually use wide backs but if I need to sew 45 inch fabric lengths together to get it wide enough, I rip two lengths of fabric and then put the selvage together and sew a 1 inch seam for a center seam. After sewing the center seam, I trim off the selvages and leave a 1/2 inch center seam. I then usually run a line of hand stitches to keep the center seam allowances to one side so they don't flip when I am quilting.