Read this - http://pubs.vmware.com/vsphere-51/topic/com.vmware.ICbase/PDF/vsphere-esxi-vcenter-server-51-setup-mscs.pdf
Basically, you add a new disk as type RDM on the first cluster node and then add that disk to your second node as an existing vmdk mapped back to the first VM. I really reccomend that you read that guide I linked as there are other considerations to take into account such as HA\DRS and anti-affinity rules for your clustered VM's.