Many times, you may have a preference of aligning some of the photos on the left page with some of the photos on the right page, then the two-page spread edit mode will be the function you need.

Click Tools top menu, then click Edit Two-Page Spread to access this function.

In this special edit mode, each of the spread pages, the current left and right pages, will be displayed on one screen, so you can adjust the positions of the photos on each side and make sure they are aligned correctly.

This is really a very helpful tool in designing a photo book.