Filter post by current 2 differents users id

Based on wordpress reference for WP_QUERY you can use author__in to filter by more than one author. It uses an array of user ids. Here is your code with the needed changes:

function list_cpt_by_author_id(){
    $user = get_current_user_id();
   
$authors_id = array("1", $user);

    
    $custom_terms = get_terms('custom-tax');
        foreach($custom_terms as $custom_term) {
            wp_reset_query();
            $args = array(
                'post_type' => 'artigos',
                'tax_query' => array(array('taxonomy' => 'custom-tax','field' => 'slug','terms' => $custom_term->slug,),),
                'author__in' => $authors_id,
            );
    
    
             $loop = new WP_Query($args);
             $image = get_field( 'cat_imagem', $custom_term );
    
             //Taxonomies Loop
             if($loop->have_posts()) {
                echo '<h2>'.$custom_term->name.'</h2>';
                echo '<img src="'.$image.'"/>';
                
                //Post loop
                while($loop->have_posts()) : $loop->the_post();
                    echo '<a href="'.get_permalink().'">'.get_the_title().'</a><br>';
                endwhile;
             }
            wp_reset_postdata(); // reset global $post;
        }
    }

P.S. I also found out that you are passing WP_USER object to $authors_id, but you only need the user’s id.