  In this theoretical paper, we investigate coherence properties of the
near-resonant light scattered by two atoms exposed to a strong monochromatic
field. To properly incorporate saturation effects, we use a quantum Langevin
approach. In contrast to the standard optical Bloch equations, this method
naturally provides the inelastic spectrum of the radiated light induced by the
quantum electromagnetic vacuum fluctuations. However, to get the right spectral
properties of the scattered light, it is essential to correctly describe the
statistical properties of these vacuum fluctuations. Because of the presence of
the two atoms, these statistical properties are not Gaussian : (i) the spatial
two-points correlation function displays a speckle-like behavior and (ii) the
three-points correlation function does not vanish. We also explain how to
incorporate in a simple way propagation with a frequency-dependent scattering
mean-free path, meaning that the two atoms are embedded in an average
scattering dispersive medium. Finally we show that saturation-induced
nonlinearities strongly modify the atomic scattering properties and, as a
consequence, provide a source of decoherence in multiple scattering. This is
exemplified by considering the coherent backscattering configuration where
interference effects are blurred by this decoherence mechanism. This leads to a
decrease of the so-called coherent backscattering enhancement factor.
