The instability of a homogeneous complex coacervate phase induced by incompatibility of oppositely charged polyelectrolytes is studied by means of the random phase approximation (RPA) and the scaling approach. It is found that in a poor solvent for both polyions their mutual repulsions may result in the formation of a microphase separated structure at low salt concentrations c(s). At high c(s, )a decomposition of the homogeneous coacervate into two macroscopic globular phases enriched either with polyanions or with polycations takes place. Both the RPA and scaling estimations predict the period of the microphase separated structure on the order of a few nanometers in the vicinity of the spinodal. The binodal of macroscopic phase separation of the coacervate and the spinodal with respect to the microphase separation are calculated, and the phase diagram of the coacervate is constructed in terms of salt concentration and incompatibility of polyions. The possibility of experimental observation of a microsegregated coacervate is discussed.