  Two microring resonators, one with gain and one with loss, coupled to each
other and to a bus waveguide, create an effective non-Hermitian potential for
light propagating in the waveguide. Due to geometry, coupling for each
microring resonator yields two counter-propagating modes with equal
frequencies. We show that such a system enables implementation of many types of
scattering peculiarities, which are either the second or fourth order. The
spectral singularities separate parameter regions where the spectrum is either
pure real or else comprises complex eigenvalues; hence, they represent the
points of the phase transition. By modifying the gain-loss relation for the
resonators such an optical scatterer can act as a laser, as a coherent perfect
absorber, be unidirectionally reflectionless or transparent, and support bound
states either growing or decaying in time. These characteristics are observed
for a discrete series of the incident-radiation wavelengths.
