A theoretical mode of the mixed alkali effect which is observed in solid electrolytes with two kinds of conducting cations is constructed with application to a quasi-2D ionic conductor. The 3D analog of this model, proposed for the first time in our previous paper [6], was based on the consideration of long-ranged Coulomb fluctuations produced by random fields of ’dopant’ and influencing on the barrier heights for migrating cations. At low concentration x (or 1 - x) of the ’dopant’ the activation energy of conductivity is shown to be an almost linear function of x (or 1 - x) for the 3D system. For a quasi-2D cationic conductor like Na/Ag beta-alumina the theoretical value of the activation energy differs from the 3D one by the logarithmic multiplier, depending on x and temperature. The application of the derived formulae to the analysis of the mixed alkali effect in quasi-2D Na/Ag beta-alumina results in good quantitative agreement between theory and experiment.