The optimal performance of a two-stage refrigeration system affected by the irreversibility of finite-rate heat transfer is analyzed by using an endoreversible combined cycle model with continuous flow. It is proven that the optimal coefficient of performance is a monotonically decreasing function of the specific cooling rate for a two-stage endoreversible combined refrigeration system. The optimal temperatures of the working fluid in the isothermal processes of a cycle are determined. The optimal distribution of the heat transfer areas are also discussed. The results obtained can provide a tool for practising engineers for designing two-stage refrigeration systems.