A hybrid system that combines a solid oxide fuel cell (SOFC) with a proton exchange membrane fuel cell (PEMFC) is presented in this paper. The SOFC stack acts as both an electricity producer and the fuel reformer for the PEMFC stack to generate additional power. A thermoeconomic model for the design optimization of a 220 kW SOFC-PEMFC hybrid system is developed in this work. Optimization of two objectives, i.e., the life cycle cost and the net electrical efficiency, are considered individually to find the optimum system configuration and component designs. Then, a multiparameter sensitivity analysis is performed to estimate the relative importance of the decision variables on the objectives. The optimization results indicate that the life cycle cost of the hybrid system is 3800–5,600 $/kW, and the maximum net electrical efficiency can reach around 63%, which is higher than an SOFC-only system, a reformer-PEMFC system, and an SOFC-gas turbine (GT) system with a similar output power. The sensitivity analysis shows that minimizing the size of the SOFC is most crucial to the system cost optimization. The hydrogen utilization factor in the SOFC is found to be sensitive to the net electrical efficiency.