So the redundancy options for ArubaOS 8 with a Mobility Master (MM) as the management platform is as follows.
First, the MM is the management platform. It does other tasks such as AirMatch, ClientMatch, AirGroup, etc. But what it does not do is terminate any APs. APs are terminated on Managed Devices (MDs) also known as Mobility Controllers (MCs).
So you have two options/environments where you may want/need redundancy. First, redundancy for the managment platform, the MM. Second, redundancy for the AP termination, so essentially redundancy of the MCs. Realistically, it is not the MCs that you are providing failover for, it is the APs. If an MC goes down, what happens to the APs, and what happens to the clients connected to the APs.
Redundancy for the MM is fairly simple. You are probably running a virtual MM, so you can simply create a 2nd virtual MM, configure VRRP between the two of them so that they have a Virtual IP (VIP). Do not forget to also enable database synchronization between them. This is part of the VRRP configuration section, but I've seen multiple organizations on large live networks not enable the database synchronizaiotn. BTW, the configuration for this is almost identical to the way it was done in earlier versions of ArubaOS. So that provides redundancy for the MM, but it does nothing for the MCs, APs, or users. So the next step is to provide redundancy for them.
The best way of providing redundancy for the MCs, APs, and users is by configuring a cluster. In the WebUI interface (logged in to the MM because that is the management platform managing all devices on the network), below the Managed Network group node, you should create a group node for your organization (not mandatory, but good practice, as I don't like making any changes at the Managed Network node). Within your organization node, you will likely have your two MCs which are known as device nodes. To start, at the group node above the devices (your organization node in this example), you will need to create a cluster profile. In that cluster profile you will specify both of the MCs as members of the cluster. After that is created, you then need to go to each of the MC device nodes and assign the cluster to the MC. It is essentially that easy.
Something else that can be helpful is to create a VRRP between the two MCs. This will allow you to point the LMS-IP of any of the AP groups you create to one address. ArubaOS 8 will distribute the APs and users across the cluster members, so a single VIP between them is fine.
I hope this helps you understand the environment,