WordPress query: merge meta key (number) values and sort

I think this is the syntax you are after:

$args = [
  'posts_per_page' => 5,
  'post_type' => 'post',
  'meta_query' => [
    'relation' => 'OR',
    'rating_au_clause' => [
      'key' => 'rating_au',
      'compare' => 'EXISTS'
    ],
    'rating_overall_clause' => [
      'key' => 'rating_overall',
      'compare' => 'EXISTS'
    ]
  ],
  'orderby' => [ 
        'rating_au_clause' => 'ASC',
        'rating_overall_clause' => 'DESC',
    ],
];

It’s a subtle difference, but uses your named meta queries in an orderby array.