  The coupled dynamics of two conjugated liquid layers of disparate
thicknesses, which coat a solid substrate and are subjected to a transverse
temperature gradient, is investigated. The upper liquid layer evolves under the
short-wavelength (SW) B\'enard-Marangoni (BM) instability, whereas the lower,
much thinner film undergoes a shear-driven long-wavelength deformation.
Although the lubricating film should reduce the viscous stresses acting on the
up to one hundred times thicker upper layer by only 10%, it is found that the
critical Marangoni number of marginal stability may be as low as if a
stress-free boundary condition were applied at the bottom of the upper layer,
i.e. much lower than the classical value of 79.6 known for a single film.
Furthermore, it is experimentally verified that the deformation of the
liquid-liquid interface, albeit small, has a non-negligible effect on the
temperature distribution along the liquid-gas interface of the upper layer.
This stabilizes the hexagonal pattern symmetry towards external disturbances
and indicates a two-way coupling of the different layers. The experiments also
demonstrate how convection patterns formed in a liquid film can be used to
pattern a second conjugated film. The experimental findings are verified by a
numerical model of the coupled layers.
