## 1 Introduction  
The metaverse [1; 2; 3] can be thought as an extension of earlier multi-user shared Virtual Reality (VR) systems. Support for collaboration is an essential feature in these systems in VR environments and has become an important topic in the field of human-computer interaction (HCI) [4; 5]. Collaboration in VR may increase the quality of shared presence, communication, and interactions among multidisciplinary teams distributed in several locations [6; 7; 8; 9; 10]. Although issues of information security have been raised for the use of the metaverse [11], these are out of the scope of the present paper. To enable truly effective collaboration in VR, one needs to find out the most important system features for the collaborative participants. [PERSON] et al. [12] studied the interplay between telepresence, interactivity, and immersion in collaborative VR and their results demonstrated the importance of interactivity and immersion; they claim that trust between users of the same team is essential for collaborative VR experiences. It was also shown that immersion is one of the main drivers. [PERSON] et al. [13] demonstrated that the roles and actions of the participants in collaborative VR will affect participant experience. [PERSON] et al. [14] studied collaborative learning systems where either both participants were using VR devices or one had a VR device and the other was using a tablet. They demonstrated that participants experienced greater presence and immersion, and lower cognitive load, when both had VR devices. [PERSON] et al. [15] demonstrated that asymmetric remote collaboration, where some participants used VR and others attended by other means, was still useful. Spatial perception is important when analyzing medical imagery for operation planning. [PERSON] et al. [16] found that the use of collaboration in the VR environment improved cognition capability. [PERSON] and [PERSON] [17] showed that the use of VR made it possible to detect possible conflicts in spatial arrangements. [PERSON] et al. [18] used VR to enable a real-time remote collaboration for the design review process and demonstrated that using an immersive VR application led to better results than using the traditional approach. Increased spatial understanding was also one of the benefits of using a collaborative VR system in a study by [PERSON] et al. [19], who studied a system for technical document review and risk assessment. In the medical field, collaboration in VR environments has been investigated in various contexts, for example in medical training where several participants working in the same environment and on the same data sets is natural. For example, [PERSON] et al. [20] studied how the training for a collaborative medical operation can be done in a VR environment and [PERSON] et al. [21] used multi-user VR and mixed environments to learn liver anatomy. [PERSON] et al. [22] collected a review about using VR for teaching in the health profession. In the learning situations, the teacher usually explains the phenomena or operation, and other participants will be observing a demonstration, but a true collaboration with active participants can also be arranged. [PERSON] et al. [23] demonstrated a system where the participants studied shared electronic patient records in VR and [PERSON] et al. [24] had a collaborative VR system where several domain experts can study the same medical data. [PERSON] and [PERSON] [25] explored collaborative bone surgery planning in a tele-immersive environment with networked Cave Automatic Virtual Environment (CAVE) systems and other stereoscopic desktop displays. To the best of our knowledge, this is the first study of collaborative VR for computer-assisted surgical simulation (CASS) in facial or cranioxillofacial surgery, while VR has earlier been used for solo training [26] and planning [27].