Hello I must split a Wordpress site by 50 percent groups.

One category is going to be proven on: www.domain1.com and also the other group of pages is going to be show on www.domain2.com. Online hosting I've aliasing on.

The present website is build with joomla and also the script that handles the best redirect is:

if($secid == 2 && ($siteurl == 'http://www.domain1.nl')){
Header( "HTTP/1.1 301 Moved Permanently" ); 
Header( "Location: http://www.domain2.nl" . $url_suffix . '/' . $url );
}

if($secid == 1 && ($siteurl == 'http://www.domain2.nl')){
Header( "HTTP/1.1 301 Moved Permanently" ); 
Header( "Location: http://www.domain1.nl" . $url_suffix . '/' . $url ); 
} 

Now I am developing this site for Wordpress. What must i do? Create a redirect within the header.php or create a multisite using the new function in Wordpress 3?