Is it possible to make a data volume visible by several VMs simultaneously?
It is not possible on a cloud level of management. It is, however, possible (and widely practised) to attach such volume to one of the VM-s and export it to others by means of networking file system, e.g. NFS, supported on operating system level. See How to export volume over NFS? . One can also use Object storage to make data available to several VMs.