The response of Rayleigh-Bénard convection in a horizontal fluid layer to time-periodic heating of its horizontal boundaries with a mixture of two frequencies is analyzed numerically. The ratio of the two forcing frequencies and the mixing angle of the amplitudes of modulation provide a control on the instability of the system. In addition to the existence of well-known harmonic and subharmonic instability responses under modulation, the time-periodic oscillation of the boundary temperatures of the fluid-layer with two frequencies results in more bicritical states in comparison to the single-frequency excitation. The onset of instability depends strongly on the modulation parameters and the Prandtl number of the fluid.