How to override Category rendering mechanism

if you can use template_redirect hook and a simple function to set the template to use

add_action( 'template_redirect', 'category_set_redirect' );
function category_set_redirect(){
    //create the sets of categories
    $first_set= array("1","2","3","4","5","6");
    $second_set= array("7","8","9","10","11","12");

    if (is_category()){
        if (in_array(get_query_var('cat'),$first_set)){
            include(TEMPLATEPATH . "/first_category.php"); // include the corresponding template
            die();
        }
        if (in_array(get_query_var('cat'),$second_set)){
            include(TEMPLATEPATH . "/second_category.php"); // include the corresponding template
            die();
        }   
    }
}