Hello Attila.
Extreme Control (NAC) acts from the switch or wireless controller point of view like a radius server.
Therefore, the typical radius redundancy mechanisms apply. If the first NAC does not respond, the second is addressed. The two NACs work independently of each other. That's why what Ron says is right.
Just install a second NAC (same way as the first one) without a special HA configuration. Via XMC (or on switch CLI) you can then configure both NACs on the switches and thus create the redundancy from the switch perspective.
Regards
Stephan
Regards
Stephan