A torque box is the thick (14ga.) box that is welded in beneath the kick panel(s) on your car. They prevent body flex that is produced when the engine turns. The cars motor turns toward the drivers side when it runs. Just rev your car while its sitting and you will see the whole car sway towards the drivers side. Convertibles have one on each side to make up for the absence of roof support (they also have thicker floor pans - which is why they are more difficult to replace). My guess why a coupe or fastback has two is because it came with a big block, but im not sure. My small block coupe has only one.