Hi there,
From experience you may set it up on either options:
1- Using the MGMT IP as your LMS and Backup LMS as long as the VLAN your access points on can reach that MGMT IP address.
2- I usually like having a dedicated WLAN VLAN that my APs are on and the controllers have an IP address on that VLAN.
I'm curious though as to why are you using Backup LMS? why not just use LMS and have your backup comes from the HA group configurations. Just a thought.
Cheers.