Three mixed crystals, kappa-(ET)(2)Ag-2x Cu2(1-x)(CN)(3) [ET is bis(ethylenedithio)tetrathiafulvalene; 0.24 < x < 0.71] with a kappa-type packing motif of face-to-face ET dimers, were obtained by electro-crystallization. Regardless of the composition, each ET dimer fits into a hexagonal anionic opening (i.e., key-on-hole packing) similar to its parent spin liquid candidate, kappa-(ET)(2)Cu-2(CN)(3). X-ray diffraction and energy dispersive spectroscopy analyses revealed that Cu and Ag atoms are statistically disordered with a fairly homogeneous distribution in a crystal. A structural variation depending on x is responsible for the change in the calculated band parameters related to intermolecular interactions, electron correlations, and frustrations. A salt with nearly equimolar amounts of Ag and Cu (x = 0.49) is semiconductive at ambient pressure and undergoes a Mott transition upon application of hydrostatic pressure. Along with the positive pressure dependence of the transition temperature, the temperature-independent amplitude of magnetic torque at low temperatures suggests that the insulating phase is a quantum spin liquid. Further application of pressure results in the appearance of a superconducting phase. Contrary to those of the parent salts, kappa-(ET)(2)Cu-2(CN)(3) and kappa-(ET)(2)Ag-2(CN)(3), the transition temperature increases as the pressure increases and eventually reaches 4.5 K at 1.65 GPa.