  We calculate the one- and two-photon scattering matrices of a pair of
collocated non-identical two-level atoms coupled to a waveguide. We show that
by proper choice of a two-photon input, the background fluorescence by the
atoms may be completely quenched, as a result of quantum interference, and that
when the atoms' detuning is smaller than their linewidths, extremely narrow
fluorescence features emerge. Furthermore, the system emits a two-photon bound
state which can display spatial oscillations/quantum beats, and can be tuned
from bunched to anti-bunched statistics as the total photon energy is varied.
