Cloud point curve, critical composition and several critical coexistence curves were measured at 25 degrees C for the ternary system poly-L-(lactic acid) (PLA), poly(methyl methacrylate) (PMMA) - where both polymers exhibit broad molecular weight distributions - and the common solvent chloroform; In contrast to the situation encountered in the absence of the second polymer both branches of the critical coexistence curves are located without any doubt inside the miscibility gap as defined by the cloud point curve. This unexpected experimental finding is corroborated by model calculations on the basis of continuous thermodynamics. The removal of solvent from the ternary mixtures yields films of different morphology, depending on the particular path through the metastable or unstable regions of the phase diagram. The structures observed by means of optical microscopy confirm the theoretically postulated phase separation mechanisms.