MirrorINI

o Installing MirrorINI extension

Extract extension to the extension/mirror_ini/ directory.

Edit settings/override/site.ini.append.php and register the similar siteaccesses like this:
[MirrorINI]
MirroredSiteAccessList[]
MirroredSiteAccessList[]=first_siteaccess
MirroredSiteAccessList[]=second_siteaccess
...

Edit all the ini files in siteaccess settings that have unique parts.
Wrap the unique parts with the following markers:
# <<< Mirror INI start
<unique settings for this siteaccess>
# <<< Mirror INI end

MirrorINI will ignore any parts of the ini files that are wrapped with these markers.

o Using the MirrorINI extension

php extension/mirror_ini/bin/mirror_ini.php --siteaccess=<siteaccess> [--file=<ini filename>] [--nobackup] [--help]

--siteaccess (-s) : Master siteaccess, the settings in this siteaccess are forced on the others
--file (-f) : Specify a single ini file to mirror between the siteaccesses (optional)
--nobackup : Does not create a backup file when editing ini files. All changes are irreversible! (optional)

If --nobackup parameter is not specified, the old content will be
copied to a new file called <ini-file>.old. Next time MirrorINI
need to update the same ini file, this backup file will be overwritten
with a new backup.