Wordpress Page über zwei Domains

// correct hardcoded urls - admin area is on "www.first-domain.de"
$other_domains = array('www.second-domain.de', 'www.third-domain.de');
if ( in_array( $_SERVER['HTTP_HOST'], $other_domains ) ) {
    function my_set_up_buffer(){
        if ( is_admin() ){
            return;
        }
        ob_start( 'my_filter_page' );
    }
    add_action( 'wp', 'my_set_up_buffer', 10, 0 );
    function my_filter_page( $html ){
        return str_replace( '//www.first-domain.de', '//' . $_SERVER['HTTP_HOST'], $html );
    }
}
// avoid "duplicate content", set main domain for indexing to "www.second-domain.de"
function my_add_canonical() {
    if ( !is_404() ) {
        $other_domains = array( 'www.first-domain.de', 'www.third-domain.de' );
        if ( in_array( $_SERVER['HTTP_HOST'], $other_domains ) ) {
            echo '<link rel="canonical" href="http://www.second-domain.de' . $_SERVER['REQUEST_URI'] . '" />';
        }
    }
}
add_action( 'wp_head', 'my_add_canonical' );