A suggestion, albeit a sorta hack, I'd like to make is to use a mailing list. You can add an infinite number of emails to a mailing list.
An alternative you could do is to use the publish_{$posttype} hook to send email notificiations through wp_mail. The wp_mail function's $to parameter takes either a string or an array so you could pass in multiple email addresses.
EDIT:
function notify_users_of_new_post($post_id) {
// Get list of subscribers and their secondary email address stored in wp_usermeta
$user_query = get_users('blog_id=1&orderby=nicename&role=subscriber&fields=all_with_meta');
$bName = get_bloginfo('name');
$permalink = get_permalink($post_id);
foreach($user_query as $user) {
$email = $user->email;
if (isset($user->secondary_email) AND !empty($user->secondary_email)) {
$email = array( $user->email, $user->secondary_email );
}
wp_mail($email, sprintf('New Blog Entry on %s', $bName), sprintf('A new entry has been published to %s. View it clicking here: %s', $bName, $permalink));
}
}
add_action('publish_post', 'notify_users_of_new_post');
I attempted a sample code block (above). A suggestion if you plan on using this on a production site, use a cron job if you have a lot of registered users, otherwise I'm pretty sure this will cause a timeout in PHP.
var_dumpthe output to make shure you ain't got the comma or white spaces? – kaiser Feb 20 '12 at 16:31