Instabilities of a pressure driven two-layer Poiseuille flow confined between a rigid wall and a Darcy-Brinkman porous layer are explored. A linear stability analysis of the conservation laws leads to an Orr-Sommerfeld system, which is solved numerically with appropriate boundary conditions to identify the time and length scales of the instabilities.fde The study uncovers the coexistence of twin instability modes, (i) long-wave interfacial mode-engendered by the viscosity stratification across the interface and (ii) finite wave number shear mode-originating from the inertial stresses, for almost all combinations of the viscosity (μr) and thickness (h r) ratios of the liquid layers. The presence of the porous layer reduces the frictional influence on the films, which significantly alters the length and time scales of the shear mode while the interfacial mode remains dormant to this effect. This is in stark contrast to the two-layer flow confined between non-porous plates where an unstable interfacial (shear) mode is observed when μr>hr2(μr<hr2). The study reveals that strength of the shear mode, (a) increases with porosity, (b) initially increases and then becomes constant with porous layer thickness, (c) initially increases then reduces with increase in permeability, and (d) reduce with increase in the stress jump coefficient at the porous-liquid interface. Moreover, the gravity expedites the destabilization of both the modes in the inclined channels as compared to the similar non-inclined channels. The parametric study presented can find important applications in enhancing heat and mass transfer, mixing, and emulsification especially in the microscale flows. © 2013 Elsevier Ltd.