Why are trailing-edge control surfaces usually split?
Same reason you have split ailerons on most big airliners. The inboard surfaces deflect at higher speeds and the outboard ones are locked, reducing the twisting (torque) forces on the wings. It also allows for finer control (in fly-by-wire cases) as well. Like a lever, a small deflection of the outboard surface results is higher rate pitch/roll (in this delta wing elevon example) compared to same deflection of the inboard surface.