zencart上的多个mysql数据库连接

I have a requirement to connect to two separate databases for a client. One db holds all the standard store data, customers, orders etc, the other is to hold around 41,000 promo codes. I've looked at PDO to connect to multiple db's as that is the new standard, but i have no idea how to achieve this when we have a configure.php file that contains the db connection info.

configure.php listed below:

define('HTTP_SERVER', 'http://localhost');
define('HTTPS_SERVER', 'https://localhost');

// Use secure webserver for checkout procedure?
define('ENABLE_SSL', 'false');

// NOTE: be sure to leave the trailing '/' at the end of these lines if you make changes!

// * DIR_WS_* = Webserver directories (virtual/URL)
// these paths are relative to top of your webspace ... (ie: under the public_html or httpdocs folder)
define('DIR_WS_CATALOG', '/');
define('DIR_WS_HTTPS_CATALOG', '/');

define('DIR_WS_IMAGES', 'images/'); 
define('DIR_WS_INCLUDES', 'includes/');
define('DIR_WS_FUNCTIONS', DIR_WS_INCLUDES . 'functions/');
define('DIR_WS_CLASSES', DIR_WS_INCLUDES . 'classes/');
define('DIR_WS_MODULES', DIR_WS_INCLUDES . 'modules/');
define('DIR_WS_LANGUAGES', DIR_WS_INCLUDES . 'languages/');
define('DIR_WS_DOWNLOAD_PUBLIC', DIR_WS_CATALOG . 'pub/');
define('DIR_WS_TEMPLATES', DIR_WS_INCLUDES . 'templates/');

// * DIR_FS_* = Filesystem directories (local/physical)
//the following path is a COMPLETE path to your Zen Cart files. eg: /var/www/vhost/accountname/public_html/store/
define('DIR_FS_CATALOG', '/');

define('DIR_FS_DOWNLOAD', DIR_FS_CATALOG . 'download/');
define('DIR_FS_DOWNLOAD_PUBLIC', DIR_FS_CATALOG . 'pub/');
define('DIR_WS_UPLOADS', DIR_WS_IMAGES . 'uploads/');
define('DIR_FS_UPLOADS', DIR_FS_CATALOG . DIR_WS_UPLOADS);
define('DIR_FS_EMAIL_TEMPLATES', DIR_FS_CATALOG . 'email/');

// define our database connection
define('DB_TYPE', 'mysql');
define('DB_PREFIX', ''); // prefix for database table names -- preferred to be left empty
define('DB_SERVER', 'localhost');
define('DB_SERVER_USERNAME', '');
define('DB_SERVER_PASSWORD', '');
define('DB_DATABASE', '');

// The next 2 "defines" are for SQL cache support.
// For SQL_CACHE_METHOD, you can select from:  none, database, or file
// If you choose "file", then you need to set the DIR_FS_SQL_CACHE to a directory where your apache
// or webserver user has write privileges (chmod 666 or 777). We recommend using the "cache" folder inside the Zen Cart folder
// ie: /path/to/your/webspace/public_html/zen/cache   -- leave no trailing slash
define('SQL_CACHE_METHOD', 'none');
define('DIR_FS_SQL_CACHE', '/enter/your/path/to/public_html_or_htdocs/and/zencart/here/zen/cache');

Digging through the php files i found this, which appears to be where the connection is created:

// Load queryFactory db classes
require(DIR_FS_CATALOG . DIR_WS_CLASSES . 'db/' .DB_TYPE . '/query_factory.php');
$db = new queryFactory();
$db->connect(DB_SERVER, DB_SERVER_USERNAME, DB_SERVER_PASSWORD, DB_DATABASE);

I looked at using:

try {
$db1 = new PDO('mysql:dbname=databas1;host=127.0.0.1', 'username', 'password');
$db2 = new PDO('mysql:dbname=databas2;host=127.0.0.1', 'username', 'password');
} catch (PDOException $ex) {
echo 'Connection failed: ' . $ex->getMessage();
}

in place of the existing:

$db->connect(DB_SERVER, DB_SERVER_USERNAME, DB_SERVER_PASSWORD, DB_DATABASE);

but i'm concerned this means that all the existing queries will need to be changed to cover the $db1, $db2 issue. Both databases are located on the same server and use identical user/pass combinations.

Any help greatly appreciated.