Go to Configuration> Wireless> AP Configuration. Edit your AP-Group that your particular access points are in. Expand AP, Expand System Profile and Click on the System Profile. In the AP System Profile, there will be the LMS (local mobility switch) and Backup LMS parameters. Put the IP address of the ap's first controller in the LMS and the Backup controller in the Backup LMS. APs that boot up in that AP group will first go to the LMS IP address, then to the backup LMS, if the first controller goes away. Assuming your other access points are in a different group, you will have to create a separate AP system profile for that AP group that I assume would have the opposite configuration.
One word about redundancy:
When your access points boot up, they look for aruba-master.<domain>. If the controller at the other end of that ip address is down, your access points will NOT be able to do anything. What you can do, is put two ip addresses in the aruba-master a-record on your DNS server, so that your APs will receive BOTH addresses and try one after the other.