COnfigure the second controller to be in same mobility group as first one (it is under the controller tab).
A good idea is to have the second controller in the same subnet as the first one. What method are you using to let APs know of the controller IP addresses? If you are using DHCP option 43, make sure that both IP addresses are being let known to them. After that, APs will use both controllers and fail over in case one fails.
If you do not use the local database on the controller (guest access for example) I would move some access points off the primary controller. Otherwise you would have to create the same username and passwords on both controllers which is really annoying. So if you have 50 access points make the primary controller the master for 25 of them and the secondary backup, and vice versa. This way you will get a better bang for your buck.
I don't worry too much about option 43 I let the APs land on the primary controller, I change the High Availability of the AP after this point as the controller information is written to NVRAM.
Transferring Crash file from standby: Login to the Active WLC in HA.
From CLI: (Cisco Controller) >transfer upload datatype crash (Cisco
Controller) >transfer upload filename (Cisco
Controller) >transfer upload mode tftp (Cisco Controller) >transfer
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