You can manually replicate the master database to the local and let the local use the local database with following command:
"aaa authentication-server internal use-local-switch"
To manually replicate you either have to export the database from the master and reimport in the local
local-userdb export <filename>
local-userdb import <filename>
or do it 1 by 1.
There is no automatic sync for this database between master and local, the sync is only there for master/standby.
If APs are connected on both master and local, there is not really any way around this.
If the master is only the backup and APs are by default on the local (or vice versa), then maybe convert to master/standby?