Okay. For that redundancy, in the AP-Group, there is an AP system profile. In the AP system profile, there is a parameter called the LMS-IP and the Backup LMS-IP. The LMS-IP should be the primary controller and the Backup LMS-IP would be the backup controller when access points cannot access the primary.
So the access points at the first site will have an AP group, that has an AP system profile that has the LMS-IP as the primary controller and the Backup LMS-IP as the backup controller. The access points at the second site will have exactly the reverse.
Things to keep in mind:
Each WLAN determines what VLAN users will be put into, using the VLAN parameter in the Virtual AP profile:
A VLAN is a configuration that is local to each controller so that VLAN number must be defined on both controllers for your WLAN to come up. In the example above, I need to make sure that a VLAN 1 is defined on both the primary and backup controllers for users to be able to pass traffic whether it is on the primary or backup. If VLAN 1 only exists on the primary, when the access points fail over, it will not come up when it attaches to the backup controller. If you configure Named VLANs, instead, and put that in the Virtual AP vlan profile, you can define the VLAN name as whatever VLAN you want on the primary and a completely different VLAN on the backup controller.
I hope it all makes sense...