Sharing a virtual disk isn't the same thing as sharing a file system. You're looking for a shared file system. If you want that, enable a file share on one machine or the other and then use the files through networking.
If you do share a virtual disk between two VMs (which can, as the others have noted, only happen when the VHDX is stored on a clustered file system) you need to then use clustering within the group of VMs to arbitrate access to the disk, so that the various VMs don't clobber each others' block writes to the disk and corrupt the file system on the virtual disk. This pretty much requires clustering software running within the set of guest VMs, which is the reason that this feature doesn't work at all for Windows 8.1, even within a VM.
It is actually possible for more than one VM to be writing to the shared VHDX simultaneously. You can enable the CSV file system within the guest cluster and it will allow multiple cluster nodes to write at the same time, as long as the writing is happening to files that already exist on disk and to regions of the files that already exist. This makes sense for workloads like SQL Server using a file server as backend storage. For other workloads, it makes more sense to avoid CSV and let one VM use the disk at a time. A Windows file server cluster will still be highly available, by the way, in that one VM can die without interrupting file service. The ownership of the disk will seamlessly shift to another member of the file server cluster.
This, by the way, points to the reason why the shared VHDX needs to be on CSV at the hosting layer. The entire feature makes no sense unless the shared VHDX is on storage that has no single point of failure.