Dewetting dynamics of immiscible liquid polymer bilayers were studied by atomic force microscopy. Morphological changes of the phase-separated liquids were correlated with friction and local elastic properties, and the dewetting pattern classified in terms of the relative bulk viscosities. Topographical atomic force microscopy measurements revealed that the dewetting process did not occur on the liquid-liquid interface as expected. Rather the process is three-dimensional with an apparent shear plane "deep" inside the tow viscosity liquid film, The depth of the shear plane is immediately established in the nucleation process which was studied two and three dimensionally by friction force microscopy and compliance measurements. Since this mechanism has not been observed previously in simple liquids, the role of entanglements of these polymers is discussed together with the application of the atomic force microscopy as a three-dimensional material property specific tool capable of studying processes below the surface.