Coherent manipulation of a two-level quantum system is of great significance for the precision measurement and quantum information processing, such as atomic clock, atomic interferometer and quantum computing. Two-photon Rabi oscillations between hyperfine ground states of rubidium atoms simultaneously driven by microwave and radio frequency (MW-RF) fields are experimentally demonstrated. Based on the calibration of the thermal relaxation process and the measurement of the microwave transition between Zeeman sublevels, the coherent oscillations of the atomic population are distinctly distinguished in the thermal relaxation process. Moreover, the dependence of the generalized Rabi frequency on the intermediate state detuning and the power of the MW/RF fields is measured and analyzed in detail. When the intermediate state detuning is large enough, the experimental results are consistent with the equivalent two-level model. Otherwise, the measured Rabi frequency deviates from the theoretical value due to the population of the intermediate state by few atoms. These achievements provide us powerful theoretical support for the coherent manipulation of two-state quantum systems.