Multiple host writes do NOT work per test results:
Ex.
$config['db']['adapterClass'] = 'XF\Db\Mysqli\ReplicationAdapter';
$config['db']['write'] = [
'host' => '192.168.10.1',
// ... username, password, dbname etc.
];
$config['db']['write'] = [
'host' => '192.168.10.2',
// ...