The two-page spread (two page one picture) is an easy concept within your Shutterfly photo book.
Simply use a single photograph layout template on each page. Pick the left and right version of the same shape template layout. With one on each page, make the opening for the photos to meet in the middle or crease of the book. By matching both sides of the layout templates, the result, is going to be a single space for one photo with the crease of the book in the middle.
Then take the photo you wish to be the full spread and put the same image on left page and the right page. This is so that half of final photo will be on one page and the other half will be on the other. The tricky part is where to pick a place in each photo to crop in half. Then like a smooth sided puzzle match up each side. This can be the hard part. Once you have decided were you want the center of the photo to lie in your two-page spread, crop your photo there. That is the place were the crease of the book would be. Then do the same on the other page and match up the other half of your photo.
Little by little, move the cropped area with your arrow to match up with the opposite page. Some photos just cannot be matched up as much as you try and you will run into proportional issues. If you use Photoshop or Photoshop Elements, matching each side of the photo would be a breeze. However, you can do this with Shutterfly too. Just play with each side by moving your photo in the crop section until you find something that works.
I have found that the simpler the photo is the easier it is to match up. Like the water pages of my book, A Summertime Swimming Adventure, the backgrounds were easy to match because they were the same color and texture.