我不能百分之百确定您想要完成什么，但您可以通过将两个字符串串联起来“搜索两个字符串”：

function checkForAa($query){

    if($query->is_search){

        $search_string = $query->get( \'s\' );

        if( stripos( $search_string , \'aa\' ) !== false){
            $new_string = str_ireplace( \'aa\' , \'xxx\' , $search_string );
        }elseif( stripos( $search_string , \'å\' ) !== false){
            $new_string = (str_ireplace( \'å\', \'aa\' , $search_string ));
        }

        if( !empty( $new_string ) ){
           // Concatenate the strings
           $query->set( \'s\', $new_string.\' \'.$search_string);
        } 

    }
}

add_filter(\'pre_get_posts\', \'checkForAa\');

$s = \'aabcd\';
$query1 = new WP_Query( 
  array( 
    \'posts_per_page\' => 3, 
    \'s\' => $s
  ) 
);
var_dump($query1->request);