Recently Augmented Reality (AR) technology has been used to develop the next generation collaborative interfaces. First results have shown the value of using AR for co-located tasks based on exocentric viewpoints. In contrast, Virtual Reality (VR) seems to offer interesting advantages for immersive collaborative experiences with egocentric viewpoints. In this paper we focus on a new area: a mixed collaboration between AR and VR environments. We present a new conceptual model of transitional interfaces that allow users to move between AR and VR viewpoints. We then describe the results of a quantitative evaluation with an AR exocentric viewpoint and a VR egocentric viewpoint for a navigational task. We also conducted a second experiment on the impact of the relationship between the interaction and visualization space in mixed collaboration. Results of these studies can provide a better understanding of how to design interfaces for multispace and transitional collaboration.