  We consider the interaction of one- and two-photon pulses in a waveguide with
two two-level systems (TLS) that are also able to interact directly either
through an exchange- or a dipole-type interaction. We focus on the system's
transport properties and show how the presence of a second TLS increases the
control options, especially when direct interactions are also allowed. We also
obtain, within a Markov (long pulse) approximation, exact results for the
nonlinear or entangled terms that arise in the two-photon case, and discuss
both their potential applications and ways to minimize their effects.
