The operation of a dual-fuel combustion engine using combustion mode-switching offers the benefit of higher thermal efficiency compared to single-mode operation. For various fuel combinations, the engine research community has shown that running dual-fuel engines in Reactivity Controlled Compression Ignition (RCCI) mode, is a feasible way to further improve thermal efficiency compared to Conventional Dual-Fuel (CDF) operation of the same engine. In RCCI combustion also ultra-low engine-out NOx and soot emissions have been reported. Depending on available hardware, however, stable RCCI combustion is limited to a certain load range and varying operating conditions. Therefore, mode-switching is a promising way to implement RCCI in practice on short term. In this paper, a model-based development approach for a dual-fuel mode-switching controller is presented. Simulation results demonstrate the potential of this controller for a heavy-duty engine running on natural gas and diesel. First, the underlying combustion model is introduced for CDF and validated using experimental data. With this model, an existing engine model is extended to simulate both CDF and RCCI. In the second part, this extended model is used for system analysis to understand the switching behavior to design a coordinated air-fuel path controller. This closed-loop controller combines static decoupling with next-cycle CA50-IMEP-Blend ratio control. A robust, mode-switching simulation result has been presented for a low load operating point. It is shown that it is possible to switch between RCCI and CDF while maintaining the same set-points. The paper is concluded with an outlook on necessary steps to make the model-based control of dual-fuel mode-switching a success on a real engine.