Misnomer: "Air Force One" is a moniker/call sign for any plane the POTUS is on.
So only one Air Force One can exist at a time. The Other one that landed is a backup plane.
Well, if you're going to be critical, I guess I can, as well...
"Air Force One" is a call sign for any USAF aircraft the president's on. If he's on a Navy or any other service's aircraft, it's not Air Force One. :)