There must be at least one wide-area heartbeat going from each cluster to every other cluster. VCS starts communicating with a cluster only after the heartbeat reports that the cluster is alive. VCS uses the ICMP ping by default, the infrastructure for which is bundled with the product. VCS configures the Icmp heartbeat if you use Cluster Manager (Java Console) to set up your global cluster. Other heartbeats must be configured manually.
Although multiple heartbeats can be configured but one heartbeat is sufficient to monitor the health of the remote site. Because Icmp & IcmpS heartbeats use IP network to check the health of the remote site. Even one heartbeat is not a single point of failure if the network is sufficiently redundant. Adding multiple heartbeats will not be useful if they have a single point of failure.
If you have a separate connection for the replication of data between the two sites, then that can be used to reduce single point of failure. Currently, Symantec only ships heartbeat agent for symmetric arrays.