  We study in this paper the efficiency of different two-photon states of light
to induce the simultaneous excitation of two atoms of different kinds when the
sum of the energies of the two photons matches the sum of the energies of the
two atomic transitions, while no photons are resonant with each individual
transition. We find that entangled two-photon states produced by an atomic
cascade are indeed capable of enhancing by a large factor the simultaneous
excitation probability as compared to uncorrelated photons, as predicted some
years ago by Muthukrishnan et al, but that several non-entangled, separable,
correlated states, produced either by an atomic cascade or parametric down
conversion, or even appropriate combinations of coher- ent states, have
comparable efficiencies. We show that the key ingredient for the increase of
simultaneous excitation probability is the presence of strong frequency
anti-correlation and not time correlation nor time-frequency entanglement.
