This paper investigates the generation of dual-band beams by modulated metasurface antennas. The developed technique combines a semi-separable model dependence of the sheet impedance (with respect to frequency and patch parameters) with the resolution of the electric field integral equation (EFIE). This allows us to express the impedance modulation at one frequency as a function of the modulation at the second frequency. Then, we can formulate the EFIE at both frequencies while referring to the surface impedance at a single frequency, still fully taking into account the frequency dispersion of the substrate. The method is illustrated through the synthesis of a circularly polarized conical beam and a broadside pencil beam at two different frequencies. Numerical validation with two different Method of Moments (MoM) codes is provided. © 2020 EurAAP.