The effect of localization of a charged particle in two-level systems driven by two laser fields has been analyzed. An analytical solution verified by direct numerical integration for a time-dependent population has been found for arbitrary field amplitudes and frequencies in the limit of small tunneling matrix element. The topology of the localization regions in the parameter space has been studied. Stable and unstable localization regions have been found. The unstable regions originate from the nonanalytical behavior of the population with respect to a small incommensurability. This phenomenon mathematically resembles phase transition instabilities in a solid state. For the stable regions in the frequency domain threshold values have been found.