A clústercluster of virtual machines across physical hosts (also known as a cluster across boxes) protects against software failures and hardware failures on the physical machine by placing the cluster nodes on separate ESXi hosts. This configuration requires shared storage on an Fibre Channel SAN for the quorum disk.
However , as is reflected in the following screenshot extracted directly from the VM ware support site, running VM ware machines with clustered disks is not supported in MultiHost deployment if Clustered cirtual disks (vmdk) are used.
Information extracted directly from VMware support site: http://pubs.vmware.com/vsphere-51/index.jsp?topic=%2Fcom.vmware.vsphere.mscs.doc%2FGUID-E794B860-E9AB-43B9-A6D0-F7DE222695A1.html
For obvious reasons then, if you cannot go for RDM disks you should guarantee that your MCS cluster runs only in one physical host.