Actually I found an article with the MyRoundCube site. This code below does the trick. Simply replace the code in the config.inc.php file where you normally put in the public and private keys with the code below. I have it working for 11 domains already perfectly.
Thanks.
$domainkeys = array(
'mydomain_1.tld' => array(
'publickey' => '...publickey_1...',
'privatekey' => '...privatekey_1...',
),
'mydomain_2.tld' => array(
'publickey' => '...publickey_2...',
'privatekey' => '...privatekey_2...',
),
);
$domain = explode('.', $_SERVER['HTTP_HOST']);
if(count($domain) > 2){
array_shift($domain);
}
$domain = implode('.', $domain);
$config['recaptcha_publickey'] = $domainkeys[$domain]['publickey'];
$config['recaptcha_privatekey'] = $domainkeys[$domain]['privatekey'];