Obama's "escape plane"

[onyx39]

I've just been reading on a spotters forum about the above 757 (or whatever it's military designation is).
Apparently it's a secondary aircraft that's based a given number of miles away from Air Force One and, she Air Force one be compromised, it's his escape plane, and will leave if necessary with Obama aboard, in other words the rest of the administration are not bothered about.
Anyone know if this is a new thing, or has there always been a back up aircraft?
I do recall Two 747's coming into Heathrow once, so it was not possible to identify which aircraft he was on, but only once, when security was massively heightened.

[onyx39]

Ledaig said:

Small point, but 'Air Force One' is not a specific aircraft, it is the call sign assigned to any aircraft which carries the President of the USA.

I know.