The controllers would share licensing.
If it's a backup master, you would do VRRP redundancy between the two controllers and point your APs to this address (lms-ip). The controllers must both be in at least one L2 segment.
For the ClearPass redundancy, you would simply put both servers in your server group and if the first one goes down, the controller will mark it out of service and use the DR one. You could also just use a virtual-ip on the ClearPass servers and point everything to that single IP. You'll also want to turn on auto promote publisher.