A Wells turbine is a self-rectifying air flow turbine employed to convert the pneumatic power of the air stream induced by an Oscillating Water Column into mechanical energy. Standard Wells turbines have several well-known disadvantages: a very low tangential force, leading to a low power output from the turbine; a high undesired axial force; usually a low aerodynamic efficiency and a limited range of operation due to stall. In the present work we investigate extensively the potential of non-symmetric airfoil blades to improve the tangential force induced by a two-stage modified Wells turbine. Since these two stages are mirrored, the system stays globally symmetric. Non-symmetric blades allow to increase considerably power output from the turbine and the global efficiency of the system. The optimization of the gap between the two stages as well as of the shape of the airfoils is considered. The automatic optimization procedure is carried out by coupling an in-house optimization library (OPAL) with an industrial CFD code (ANSYS-Fluent). The multi-objective optimization taking into account both tangential force coefficient and efficiency relies on Evolutionary Algorithms. Detailed comparisons are finally presented between the optimized design performance and the classical monoplane Wells turbine using symmetric airfoils, demonstrating the superiority of the proposed solution.