Last year this was supposed to be the last time this happened, but the launch to the Hubble was delayed until May. So one more (and probably last time ever) there are two shuttles on the pads. No, this isn't going to be a Bruce Willis "Armageddon" launch, but just a second shuttle in case there is trouble.
Atlantis will make a final flight to the Hubble telescope for upgrades, and since there isn't a way out (like at the ISS) Endeavour is parked on the other pad ready to go rescue the crew with a few day's notice. STS-400 is the mission - and one that hopefully won't fly. If Endeavour flies, it's going up to rescue the crew of STS-125, and that is probably an early end to the shuttle program since the disabled shuttle will be dropped into the ocean.
(Click the image of the shuttles for a nice wallpaper sized image for your computer)