We propose the concept of terahertz (THz) photomixing enabled by the interband electron transitions due to the absorption of modulated optical radiation in double-graphene layer (double-GL) structures and the resonant excitation of plasma oscillations. Using the developed double-GL photomixer (DG-PM) model, we describe its operation and calculate the device characteristics. The output power of the THz radiation exhibits sharp resonant peaks at the plasmonic resonant frequencies. The peak powers markedly exceed the output powers at relatively low frequencies. Due to relatively high quantum efficiency of optical absorption in GLs and short inter-GL transit time, the proposed DG-PM operating in the resonant plasma oscillation regime can surpass the photomixers based on the standard heterostructures .