If your blocks are identically sized, you can line them up. This is how I did it. First make sure that your tetanus shots are up to date.
Take the backing square and put a thumb tack through each seam intersection that you want to match up, working from the front of the block, so the point comes out the back.
Lay the block on a flat surface and CAREFULLY smooth your batting over, being careful not to stick youself!
Now lay the top over the batting. The thumb tacks will allow you to match the intersections. Pin the heck out of it so it won't shift when you quilt it then remove the tacks.
Again, this will only work if the 2 blocks are really close in dimension.
I used this method to center a motif on a pieced back with the front of the quilt.