In mixed reality (MR) telepresence applications, the differences between participants' physical environments can interfere with effective collaboration. For asymmetric tasks, users might need to access different resources (information, objects, tools) distributed throughout their room. Existing intersection methods do not support such interactions, because a large portion of the telepresence participants' rooms become inaccessible, along with the relevant task resources. We propose MRUnion, a Mixed Reality Telepresence pipeline for asymmetric task-aware 3D mutual scene generation. The key concept of our approach is to enable a user in an asymmetric telecollaboration scenario to access the entire room, while still being able to communicate with remote users in a shared space. For this purpose, we introduce a novel mutual room layout called Union. We evaluated 882 space combinations quantitatively involving two, three, and four combined remote spaces and compared it to a conventional Intersect room layout. The results show that our method outperforms existing intersection methods and enables a significant increase in space and accessibility to resources within the shared space. In an exploratory user study (N=24), we investigated the applicability of the synthetic mutual scene in both MR and VR setups, where users collaborated on an asymmetric remote assembly task. The study results showed that our method achieved comparable results to the intersect method but requires further investigation in terms of social presence, safety and support of collaboration. From this study, we derived design implications for synthetic mutual spaces.