Yes N+1 can work between diff WLC as well.
N+1 HA means you need to configure both WLC with exact same configuration except IP and Hostname.
Do the AP groups also require the same members with the same wlan ID's and settings?
AP group name must be same. Try to keep all in identical manner on both WLC.
Do flex connect groups also need to be setup identically on both controllers?
Yes.
Regards
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