This paper describes how the rapid and detailed modelling and simulation methods can be used to increase the efficiency of mixed simulation projects initiated to support the design of interconnected ICT (Information and Communication Technology) and BP (Business Process) systems. A system of cooperating rapid and detailed methods for critical and non- critical parts of systems is introduced in the phase of preliminary and detailed modelling. The method of translation of information of conceptual models that had been built prior to simulation is described, too. New methods using rapid models to model the ICT and BP systems functioning as environment for the detailed models are presented. A novel method for preliminary modelling based only on cooperating system of rapid models is described. © 2008 EUROSIS-ETI.