For the implementation of high-gain dual-band transmit-arrays, adequate unit-cells have to be designed. The potential difficulty is related to the independent 360º phase wrapping needed at each band to compensate arbitrarily large equivalent path length phase compensation. This points to an intractable number of phase combinations at each band, leading to a possibly very large number of different unit-cells. A generic formulation is presented for the design of dual-band unit-cells that considerably reduces the search space leading to thinner and lighter configurations. An example, is presented for Ka-band with a configuration of dual-band unit-cells working simultaneously at 20 and 30 GHz. The unit-cells ensuring both the required phase values and a transmission coefficient amplitude better than -0.9 dB in both bands.