In modeling and simulation of real building and heating, ventilating, and air-conditioning (HVAC) system configurations, it is frequently found that certain parts can be represented in one simulation software, while models for other parts of the configuration are only available in other software. However, very often quality assurance dictates that the configuration should be modeled in an integrated fashion due to thermodynamic and other interactions between the various subsystems. This paper gives an overview and presents initial results of an ongoing research project that aims to improve the applicability and use in a design context of building performance simulation software by addressing this problem. The objective of the work is to research, enable, and validate run-time external coupling of distributed building and system simulation software. Although the research uses specific simulation environments, the coupling mechanism(s), data-exchange procedure, and protocol(s) that are being developed ensure that the approach has a wide and general applicability.