A shock-timing experiment plays an important role in inertial confinement fusion studies, and the timing of multiple shock waves is crucial to the performance of inertial confinement fusion ignition targets.We present an experimental observation of a shock wave driven by a two-step radiation pulse in a polystyrene target.The experiment is carried out at Shen Guang 11 Yuan Xing (SGNYX) laser facility in China, and the generation and coalescence of the two shock waves, originating from each of the two radiation steps, is clearly seen with two velocity interferometers.This two-shock-wave coalescence is also simulated by the radioactive hydrodynamic code of a multi-1D program.The experimental measurements are compared with the simulations and quite good agreements are found, with relatively small discrepancies in shock timing.