In split Cisco ISE deployments, you continue to maintain primary and secondary nodes as described in a small Cisco ISE deployment. However, the AAA load is split between the two Cisco ISE nodes to optimize the AAA workflow. Each Cisco ISE appliance (primary or secondary) needs to be able to handle the full workload if there are any problems with AAA connectivity.
Neither the primary node nor the secondary nodes handles all AAA requests during normal network operations because this workload is distributed between the two nodes.
https://www.cisco.com/c/en/us/td/docs/security/ise/2-
4/install_guide/b_ise_InstallationGuide24/b_ise_InstallationGuide24_chapter_00.html