I assume that you want to create SQL 2012 Cluster VMs across the Physical hosts.Please see the steps as below.
I think There is a typo mistake in the documents you will not get again the same option "Physical as the compatibility mode".The disk is already created with Physical as the compatibility mode on VM-1.Now you have to go with the "select the Mode: Persistent" on vm-2.Once you are done with VM-2 , Right click on VM-2 ,select the hard disk and observe compatibility mode is selected Physical only.
RDMs disks can be stored on the same shared datastore where vms are created or on the separated shared datastores.As your wish where you want to keep.Only one thing need to be remembered both datastores must be shared with the both Physical Hosts.
Thanks,
Atul