Yes it is.  It's a designed from day one to be a cluster.  You don't have to worry about one NAS head overwritting the other.   When you are using the FS7600 that's not an iSCSI target, it's a network share.  Either via NFS or CIFS.  Which multiple systems can connect to.    If you create an iSCSI volume on the array itself, then your hosts need to be running a cluster aware filesystem, that's true. 
I ask because I have multiple servers that need to write to the same iSCSI target.  As I understand it that can mean locking issues and corruption.  This is rectified by a cluster aware file system.  I see a lot of mention of the word "cluster" with the FS7600, but I do not know if Dell's FluidFS is a cluster aware file system or if the FS7600 is somehow a cluster os in general.  Can the FS7600 help me in my concerns over corruption or do I still need to have some other clustered file system.
Is Dell's FluidFS a cluster file system?