For a cluster, you'll need at least 2 VM's. (preferably on two different boxes, putting them both on one box is nice for playing around, but it brings you nothing than more points of failure in a production environment). Next you will need to configure a shared disk, accessible by both machines. This is usually based on some hardware contraption, such as a NAS, or a firewire setup. You may also be able to SIMULATE this in VMWare ESXi, consult the product's documentation.