To share the users list in a “farm” setup in DokuWiki, meaning when you have multiple wikis running off of one codebase, you must add this code:

$config_cascade['plainauth.users'] = array(
    'default'   => '/your/path/to/conf/users.auth.php',
);

to inc/preload.php and it must come after this line:

include(fullpath(dirname(__FILE__)).'/farm.php');