Are you talking about HDFS Federation ?
It is slightly different from standard NameNode HA in which one Active / one Standby NameNodes are used.
The federation uses multiple independent Namenodes/namespaces. The Namenodes are federated; the Namenodes are independent and do not require coordination with each other. The Datanodes are used as common
storage for blocks by all the Namenodes. Each Datanode registers with all the Namenodes in the cluster. Datanodes send periodic heartbeats and block reports. They also handle commands from the Namenodes.
Following is a good blog to go through: