To enable solid-state batteries with Li metal anodes, different classes of electrolytes are being investigated, mainly polymers, oxides, and sulfides. To overcome individual drawbacks of the material classes, so called bilayer hybrid cells combining layers of different electrolyte materials are considered highly promising. However, the introduction of an additional interface between the two solid electrolytes raises the question of the corresponding interfacial resistance. Herein, the properties of the interface between the two promising electrolytes Al-LLZO (Li_6.16Al_0.28La₃Zr₂O₁₂) and LPS (Li₇P₃S₁₁) are investigated by comprehensive impedance analyses. A series of systematic measurements allows to deconvolute the pressure dependence of the resistances related to the LPS material and the LPS/LLZO interface. This analysis reveals a significant pressure dependence of the LPS/LLZO interfacial resistance, mainly attributed to constriction resistance. Optimization of the LPS/LLZO interface is carried out by targeted surface treatment, resulting in a dramatic decrease of the interfacial resistance to virtually zero. These mechanistic studies and optimization approaches are complemented by practically relevant cycling tests using Li-metal electrodes. The cells show excellent cycling stability and negligible degradation. These findings contribute to the fundamental understanding of solid-solid interfaces in bilayer hybrid cells, indicating that solid-state batteries with hybrid LPS/LLZO electrolytes appear highly promising.