  We study the non-linear current response of a Dirac model that is coupled to
two time-periodic electro-magnetic fields with different frequencies. We
distinguish between incommensurable and commensurable frequencies, the latter
characterized by their ratio p/q with co-prime integers p and q. Coupling the
(effective) two-level system to a dissipative bath ensures a well-defined
long-time solution for the reduced density operator and, thus, the current. We
then analyze the spatio-temporal symmetries that force certain current
components to vanish and close with conclusions for directed average currents.
