It can run on both, the standby will take over. The license is shared between cluster members.
From WiNG 5.8.2 clustering is not supported and it is recommended to use database replica set for HA but the licensing is pretty much the same, it only needs to be installed on the primary member and it is shared between other members.
My understanding is that multiple instances of NSight can be run as a database replica set as mentioned by Vedran. However the number of NSight instances needs to be an odd number (3,5,7,9) in order that one of the instances can act as an arbiter between the other databases.