I have the same scenario as yours: ise1 is primary Admin/MNT and ise2 is secondary Admin/MNT. ise1 ip address is 192.168.1.1/24 and ise2 is 192.168.1.2/24. They are both on the same subnet.
simulate a disaster: shutdown the switchport that ise1 is connected to.
1- manually promote ise to primary Admin/MNT. After that make a bunch of changes to ise2.
2- bring back ise1. At this point, both ise1 and ise2 are shown as Primary Admin
3- from the WebUI in ise2, highlight ise and hit the button "sync-up". That will force ise1 to become Secondary Admin
4- Once everything is sync'ed, log into the ise1 WebUI and manually promote ise1 to be Primary Admin/MNT again.
Does that make sense?