We present here an experimental investigation into the optical inhomogeneities that may appear in a three-electrode, dual discharge XeCl-laser operated in the regime with pre-multiplication of electrons. Different discharge behaviors in two laser channels have been revealed and explained. It was found that the onset of output power fall off correlates with the development of discharge instabilities. Based on the data obtained the conclusion about the development and evolution of filamentous channels in the discharge plasma is drawn.