Concorde is not the only airplane with this design. It is very common on modern heavy jets. We first saw the concept with split ailerons as the wings are much bigger then the tail but we now use the same concept on the tail as well, both for the rudder and the elevators.
There are several reasons for split control surfaces. Of course it provides redundancy as each surface can be controlled by a separate system. But you also tend to see them move independently with the control surface further from the body deflecting less then the one closer to the body. This is because this control surface apply a lot more stress on the tail and could potentially cause metal fatigue if used at high speed or with full deflection. It also induces more roll then the control surface closer to the body. At high speed you also need much less authority so limiting the control to one surface give you better fine control as you have less control surface area. This is also why a split rudder was a must have for the Concorde while other airplanes designed at the same time went with a single rudder. But you find split rudders on many heavy jets.
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