  Mapping equations of motion of the highly exited classical atom in a
monochromatic field are generalized for the two-frequency microwave field.
Analysis of the obtained equations indicates to the weak sensitivity of the
position of the recently observed ionization peak near the main resonance to
the frequency and amplitude of the additional microwave field. In the high
frequency region, however, the sensitivity of the enhanced ionization peaks on
the additional field frequency is predicted.
