Is it possible to join multiple ClearPass servers(subscribers) to the publisher simultaneously?
It is recommended to join one node at a time to the publisher/cluster, since the ClearPass publisher will lock the database temporarily for further configuration changes when one the node is in the process for synchronizing the configuration from publisher(cluster joining operation). The lock will be released after the node(subscriber) successfully joined to the cluster.
Therefore, the cluster join will fail if you try to add multiple nodes to the publisher simultaneously, as the DB will be in locked state for the first node that made entry as subscriber.
Please find the below screen captures, confirming the temporary database lock when one of the node is joining to the Publisher. You need to wait for the release of database lock to join another node to the cluster.