High-intensity laser solid interactions are capable of generating attosecond light bursts via high-harmonic generation-most work focuses on single beam interactions. In this paper, we perform a numerical investigation on the role of wavelength and polarization in relativistic, high-harmonic generation from normal-incidence, two-beam interactions off plasma mirrors. We find that the two-beam harmonic-generation mechanism is a robust process described by a set of well-defined selection rules. We demonstrate that the emitted harmonics from normal-incidence interactions exhibit an intensity optimization when the incident fields are of equal intensity for two-color circularly polarized fields.