  Near-resonant ac-drive acting on a two-level system induces the Rabi
oscillations of the level occupations. It is shown that additional weak drive
properly frequency-detuned from the primary drive causes a resonant response.
This response manifests itself in the emergence of the envelope of the
oscillations. At resonance, the inverse period of the envelope is proportional
to the amplitude of the weak drive. The resonant condition reads: difference of
frequencies between the two drives is equal to the ac-splitting of quasilevels
in the field of the strong drive. Technically, the resonance can be inferred
from the analogy between the equations for the time-evolution of the spin
amplitude and the Mathieu equation, which describes e.g. the parametric
resonance.
