Two urls for one website

1. Move the wordpress site to his new domain.
2. Make sure everything is working, hint: update all url’s using Velvet Blues Plugin
3. Point the old domain to a directory containing only two files:

A file named “index.php” containing:

<?php
$goto = $_SERVER["REQUEST_URI"];
header("HTTP/1.1 301 Moved Permanently"); 
header( "Location: http://new-domain-name.ch" . $goto );
?>

And a file named “.htaccess” containing:

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

Now all traffic will be redirected to your new site, telling google to transfer the old sites value.

Update: Canonical link rel

You could also add this code to your existing sites functions.php (inside your theme), instead of completing step 3.

add_action('wp_head','rel_canonical');

function rel_canonical()
{
$goto = $_SERVER["REQUEST_URI"];
$output = "<link rel="canonical" href="http://new-domain-name.ch/$goto" />";

echo $output;
} 

This will dynamically output canonical links to all your WordPress pages.