How to tie two conditions to one statement

You can do this:

$ac_inc = get_the_author_meta( 'ac_inc', $post->post_author );
if (has_tag('aria condizionata', $post) && !empty($ac_inc)) { 
   $ac_inc="<div class="ac_inc">". $ac_inc .'</div>';
}

But actually the double “if” statement would be more optimal, as you would only run ‘get_the_author_meta’ when needed:

if (has_tag('aria condizionata', $post)) { 
   $ac_inc = get_the_author_meta( 'ac_inc', $post->post_author );
   if(!empty($ac_inc)){
        $ac_inc="<div class="ac_inc">". $ac_inc .'</div>';
   }
}