Wave formation in the gravity-driven low-Reynolds number flow of two liquid films down an inclined plane is studied by a linear stability analysis. Wavy motion can appear due to an instability of either the fluid-fluid interface or the fluid-air free surface. It is shown that the flow is always unstable and wavy motion can occur when the less viscous layer is in the region next to the wall for any Reynolds number and any finite interface and surface tensions. Stability can be achieved for the configuration with the more viscous component adjacent to the wall in the presence of interfacial tension when Reynolds number is small enough.