Oscillating Water Column (OWC) has attracted significant research efforts in the field of wave energy technology in the last decades. Recently, the Twin Turbines Configuration (TTC) has received attention because it is based on the use of unidirectional turbines which are much more efficient. Nevertheless, its main disadvantage with respect to classic OWC systems equipped with bidirectional turbines is the duplicity of equipment. To overcome this limitation, the Double Decker Turbine (DDT) concept is presented here as a new turbine which combines the best of both solutions, the TTC and the bidirectional turbine, in which the geometrical design of the DDT has been carried out based on a well-tested geometry taken from the bibliography. In addition, CFD modelling has been used to obtain an optimized DDT by means of a comparison between two new geometries for the outer turbine; while the inner turbine geometry has been already taken from the literature. Outer turbine has been further validated through experimental measurements of a manufactured prototype. Finally, a non-steady analysis of the proposed double turbine is carried out assuming sinusoidal flow conditions. Results demonstrate that the DDT concept can compete with the other alternatives, even envisaging high-performance characteristics to be developed in future works. (C) 2020 Published by Elsevier Ltd.