Combinatorial thickness-gradient libraries are used to explore stability and dewetting in thin bilayers of polystyrene (PS) on poly(3-octylthiophene) (P3OT) on Si. At low P3OT thicknesses (90-175 nm), P3OT screens the unfavorable van der Waals (VDW) interactions between PS I and Si, and the instability onset thickness of PS is depressed from 56 to 23 nm. A dramatic and unexpected transition to a VDW-unstable interface is discovered at P3OT thicknesses above 175 nm.