The 2 presidential B747's always travel together. Both aircraft are usually fully loaded with people (press, aides, security people) and other equipment (e.g. his marine helicopter, his bullet proof cars, etc..). While both are usually required for normal presidential travel, either one will either be a decoy and/or a backup. If there are mechanical problems, the president would depart on the other plane and the rest would just have to stay behind (but you can't ask the president to stay behind or wait, obviously). Also, it functions as a decoy, because when approaching the airport for landing, "terrorists" would hardly know if the first aircraft has the president on board or the second one.
As for Airforce 2, thats any aircraft the vice president travels on. Beyond the vice-president, there are no designations.