It (master/backup master) is supported on 6.4 on onwards. If you have redundant masters, just pointing APs at the VRRP is more straightforward.
If you want to do it (HA/fast failover on master/backup master), this is how you would do it. In this example, 10.10.1.1 is the master and 10.10.1.2 is the backup master. These are the controller-ips, and not VRRP or loopback addresses.
On the Master:
ap system-profile "primary"
lms-ip 10.10.1.1
bkup-lms 10.10.1.2
!
ha group-profile "Cluster-A"
preshared key presharedkey
state-sync
controller 10.10.1.1 role dual
controller 10.10.1.2 role dual
!
ap-group "Cluster-A"
ap system-profile "primary"
!
ha group-membership Cluster-A
On the Backup Master:
ha group-membership Cluster-A
backup-lms is required in case of controller failure and AP reboot.
NOTE: Inter-Controller heartbeat is not needed/supported in master/backup master deployment. The APs should be pointed at the VRRP for master discovery (aruba-master, DHCP option)
If you are terminating APs on a master/backup master pair, it is much more straightforward just to point the APs at the VRRP using your discovery method and in the LMS-IP.