  We study the effect of unidirectional reflectivity in the bilayer optical
model with the balanced loss/gain terms. It was found that if the impedances
characterizing the scattering part are different from those describing the
leads, a new effect of double-sided unidirectional reflectivity emerges. In
this case the vanishing of reflectivity occurs either for the left or right
reflectivity, depending on the frequency of scattering wave. It is shown that
the alternation of the unidirectional reflectivity from the left to the right
can occur even if the corresponding Hamiltonian is non-PT-symmetric. Our
analytical results are demonstrated by numerical data.
