It is not explained in the documentation properly, but I did some research regarding the backup of active/active cluster.
What you need to do:
1. Install NBU client to both nodes using nodenames as client name (i.e. FC01-A and FC01-B in your case)
2. DO NOT specify any REQUIRED_INTERFACE (it useless as it's impossible to guess which virtual IP will be owned by a node)
Next we need to introduce Virtual hostnames to NBU. In NBU these entities are called as app_clusters:
In this example we will create two app_clusters:
nbemmcmd -addhost -machinename APP1 -machinetype app_cluster
nbemmcmd -addhost -machinename APP2 -machinetype app_cluster
Now we need to add nodes to these clusters:
Cluster APP1, substitute master_name with your master's hostname
nbemmcmd -updatehost -add_server_to_app_cluster -machinename FC01-A -machinetype client -clustername APP1 -netbackupversion 7.0 -masterserver master_name
nbemmcmd -updatehost -add_server_to_app_cluster -machinename FC01-B -machinetype client -clustername APP1 -netbackupversion 7.0 -masterserver master_name
Cluster APP2, substitute master_name with your master's hostname
nbemmcmd -updatehost -add_server_to_app_cluster -machinename FC01-A -machinetype client -clustername APP2 -netbackupversion 7.0 -masterserver master_name
nbemmcmd -updatehost -add_server_to_app_cluster -machinename FC01-B -machinetype client -clustername APP2 -netbackupversion 7.0 -masterserver master_name
Basically, that's it. Now you can use APP1/APP2 as a client name if you want to backup shared resource and NBU will reroute your request to corresponding node which owns the IP and FC01-A/FC01-B if you want to backup something on the node level (like System_State).
This seems to be only solution that allows to backup active/active clusters, I don't know why it is not documented at all.