Air Force 2
It is sitting there powered up for a 2 min getaway, crew in place, 24/7. Doubt the engines are running but definately powered up ready to start. Just a backup, guess it makes sense to have some redundancy not at the same airport.
I guess it needs to be explained once more.
"Air Force One" and "Air Force Two" are not aircraft, they are radio callsigns. They can be used by any aircraft in the USAF fleet carrying the President or Vice President respectively.
The two VC-25's which use these callsigns are known as SAM 28000 and SAM 29000 when not carrying the appropriate VIP.
Where both of them are away on the same trip one will be Air Force One only when the President of the USA is on board, if he's not on board it will be SAM28000 or SAM29000. The one not carrying the President will always be SAM2x000.
So...Air Force 2 is not sitting there all powered up, SAM 2x000 is.