Space Shuttle question

This NASA image was take a few weeks ago at the Kennedy Space Center and shows two shuttles sitting on launch pads. The Shuttle in the foreground (on the left in this image) is Atlantis, which lifted off on Monday afternoon as STS-125. The second shuttle in the background is Endeavor:
My question is this: why are there TWO shuttles ready to launch?
And no, it doesn't have anything to do with this scenario: