Actually, most things that fly faster than the speed of sound create two sonic booms. It's just that they're usually so close together that they sound like just one. The first one is created at the front of the plane, where the nose presses on the air it runs into. The second is made at the rear, where the tail leaves an empty space behind it. At each end, the air pressure is strongly changed by the plane, creating sound waves.
You can think of the waves coming from the nose and tail as being two cones, separated by the length of the plane. the time between when you hear each wave is the time it takes for the plane to fly its own length. Since planes are fairly small, that time is short and your mind treats the double wave as a single sound. Space shuttles are a lot bigger, so you can notice the double wave.
Check out this link to a site by Nova
, where you can hear an actual recording of the double sonic boom from a space shuttle!
-Tamara (mods by mw)
(republished on 07/19/06)