Fluid–structure interaction of flexible splitter plates attached to two rigid square cylinders, placed side-by-side in a uniform flow is studied. A partitioned approach is used to solve the fluid and structure problems. The results from the implementation are compared with earlier results on standard benchmarks. The Reynolds number based on the edge of the square cylinder is 100. The flexibility of the splitter plates is varied. Initially, the splitter plates undergo small amplitude oscillations that are out of phase relative to each other. The fully developed unsteady response shows in-phase variation. The frequency of the initial out-of-phase vibrations is found to be greater than the natural frequency of the structure. It approaches the natural frequency with increase in stiffness of the structure. The amplitude of the response of the structure is large when the dominant vibration frequency is close to its natural frequency. Lock-in/synchronization is observed for certain values of flexibility considered.