Electron Cyclotron Resonance Heating by injection of two microwave beams with different k// has been investigated in the GAMMA 10 tandem mirror. In experiments, measurements of an effective temperature of an end loss electron flux and an end plate potential indicate that, for the same total incident power, heating efficiency of two beam heating is a little worse than that for one beam heating. Numerical analysis of electron energy gain by two beam ECRH gives the result of a little worse heating efficiency agreeing with that of the measurements, and the result is able to be explained as spatially sequential heating due to Doppler-shifted resonance gap for two beam heating.