If the NAS boxes - which I assume are supported for ESXi 5 (see http://www.vmware.com/go/hcl) - do not provide some kind of failover you may use the secondary NAS as a replication target only and only connect the hosts to it if the primary NAS fails for good.
With vSphere, setup vCenter Server and create a Cluster with both hosts and HA enabled. Configure the two hosts, so they both have access to the primary NAS and the same LUNs. With this setup you can now create the VMs and since the VM's files are located on the shared storage, it doesn't matter on which host they run. HA will take care of a restart on the surviving host in case of a host failure, i.e. register the VMs to the other hosts and power them on. There's no need to place all VMs on a single host and use one as failover. All hosts ion the cluster can be used for load sharing.
As mentioned in my first reply, for this setup, I recommended to have 6 physical NICs. Two of them dedicated to iSCSI traffic, two for VM traffic and 2 for Management/vMotion.
André