If running ArubaOS 8, when operating in Master Controller Mode (MCM), meaning one controller is operating as the Master and one or more other controllers are operating as Locals, the Master controller cannot terminate APs. So, only one controller can terminate APs, the Local controller. If it fails, there is not redundancy.
This was not the case in ArubaOS 6. In ArubaOS 6 the Master could terminate APs. ArubaOS 8 was a major architecture change, and is best run with a dedicate management server, a Mobility Master (MM), which can be a hardware version or a VM version.
If you want both controllers terminating APs, you could configure a Mobility Master (MM), which is a management server (which would need licensing), and then both controllers could be set up as Mobility Controllers (MCs). For redundancy, the two controllers could be configured as a cluster, which would provide stateful failover for any clients.