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.