This paper presents a novel middleware for developing flexible interactive multi-surface applications. Using a scenario-based approach, we identify the requirements for this type of applications. We then introduce Substance, a data-oriented framework that decouples functionality from data, and Shared Substance, a middleware implemented in Substance that provides powerful sharing abstractions. We describe our implementation of two applications with Shared Substance and discuss the insights gained from these experiments. Our finding is that the combination of a data- oriented programming model with middleware support for sharing data and functionality provides a flexible, robust solution with low viscosity at both design-time and run-time.