  We study a system of two interacting, non-indentical quantum emitters driven
by a coherent field. We focus on the particular condition of two-photon
resonance and obtain analytical expressions for the stationary density matrix
of the system and observables of the fluorescent emission. Importantly, our
expressions are valid for the general situation of non-identical emitters with
different transition energies. Our results allow us to determine the regime of
parameters in which coherent two-photon excitation, enabled by the coherent
coupling between emitters, is dominant over competing, first-order processes.
Using the formalism of quantum parameter estimation, we show that the features
imprinted by the two-photon dynamics into the spectrum of resonance
fluorescence are particularly sensitive to changes in the distance between
emitters, making two-photon excitation the optimal driving regime for the
estimation of inter-emitter distance. This can be exploited for applications
such as superresolution imaging of point-like sources.
