Is it possible to make a data volume visible by several VMs simultaneously on WEkEO Elasticity
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.
One can also use Object storage to make data available to several VMs.