  Multiplication of spin qubits arises at double resonance in a bichromatic
field when the frequency of the radio-frequency (rf) field is close to that of
the Rabi oscillation in the microwave field, provided its frequency equals the
Larmor frequency of the initial qubit. We show that the operational multiphoton
transitions of dressed qubits can be selected by the choice of both the
rotating frame and the rf phase. In order to enhance the precision of dressed
qubit operations in the strong-field regime, the counter-rotating component of
the rf field is taken into account.
