If you update your wp-config.php file with the following code
define('WP_SITEURL', 'http://' . $_SERVER['HTTP_HOST']);
define('WP_HOME', 'http://' . $_SERVER['HTTP_HOST']);
You're server will take care of the rest. The only thing to note is that if you have links to either of the domains within posts and pages, you may find yourself hopping between domains.
Make a backup of your wp-config.php file before you do the above though, just in case things get weird.