For cluster systems in vCloud the only option I have seen that is supported, is to have an external filer provide the shared disk.
IE. Virtual Filer inside the vApp (attach to a private vApp network and link the 2 or more systems to it) or a filers outside on another network that can be used.
We have done with with NFS and iSCSI with a lot of success. I also know from testing a disk can be added via vSphere API's to a multiple vCloud VM and it works - however as I understand this is not a supported configuration.
