Install client to both nodes using their node names as regular clients (do not choose "perform clustered installation of NetBackup" during the installation)
Once you will have them installed, go to your master server and run these commands:
nbemmcmd -addhost -machinename virtual_name -machinetype app_cluster
where virtual_name is the virtual name of the application.
nbemmcmd -updatehost -add_server_to_app_cluster -machinename node_name -machinetype client -clustername virtual_name -netbackupversion 6.5 -masterserver master_name
where virtual_name is the virtual name of the application.
For more information see
Veritas NetBackup (tm) 6.5 High Availability Guide ftp://exftpp.symantec.com/pub/support/products/NetBackup_Enterprise_Server/290237.pdf
Chapter
Enhanced media server configuration
The only difference you need to make sure that you mention "client" and not "media" in the nbemmcmd -updatehost