This paper presents initial results of research aimed at developing methods and tools for multidisciplinary collaborative development of dependable embedded systems. We focus on the construction and analysis by co-simulation of formal models that combine discrete-event specifications of computer-based controllers with continuous-time models of the environment with which they interact. Basic concepts of collaborative modelling and co-simulation are presented. A pragmatic realisation using the VDM and Bond Graph formalisms is described and illustrated by means of an example, which includes the modelling of both normal and faulty behaviour. Consideration of a larger-scale example from the personal transportation domain suggests the forms of support needed to explore the design space of collaborative models. Based on experience so far, challenges for future research in this area are identified.