Yes it's possible. You are talking about an N+1 situation. You can do this in the Configure>Access Point> Access Point Policies, enable "Limited ZD Discovery" set the "Prefer Primary ZD" and configure the AP to only connect to two ZD IP address/name for example (a 3 ZD environment, but it can be 2 or more):
Controller 1 = 192.168.1.100
Controller 2 = 192.168.2.100
Controller 3 = 192.168.3.100
Controller 1, AP Policies:
Controller 2 AP Polocy:
Controller 2: Policy:
Do not set, this way the AP's list of Primary and Secondary ZD is intact when it's on the third controller so I can go back to the "Preferred Controller"
You will have to make sure that config from controller 1 and 2 are on controller 3, wlan/wlangroup/AP/AP group, so that if an AP from controller 1 or 2 somehow connects to the "secondary"/backup controller it will be able to join, put in the correct AP group and use the appropriate wlans. Just make sure that you have enough license on the backup controller to accommodate the APs. If controller 1 has 50 AP and controller 2 has 100 AP and you want the backup controller to be able to handle the load if BOTH controller 1 and 2 does, you will need to make sure controller 3 has 150 AP license. If you expect that only one will ever fail at any given time then you only need 100 AP license so if controller 2 fails, all the APs can be managed.