There is a backup jet which will sometimes follow AF1, codenamed E-4B. The auxiliary jumbo jet is there to take over for any reason if needed. This is a common approach found in many Air Force and NASA missions. Always have a backup if you can.
The second 747-200B which sometimes trails AF1 isn’t only a backup jet. It’s the exact same craft in every way, six stories tall, ready for action. In fact, the president can fly in either one at any time.