How to do a wp_query with two acf-fields, sorting on one of them

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You are just confusing the array levels, ordering is not done inside the meta_query level but on the main query level. It is the difference between two classes:

https://codex.wordpress.org/Class_Reference/WP_Query
https://codex.wordpress.org/Class_Reference/WP_Meta_Query

Also, as the meta query codex says: “Custom field value can be an array only when compare is ‘IN’, ‘NOT IN’, ‘BETWEEN’, or ‘NOT BETWEEN'”… I think you really only need NOT EQUALS here to check footnote value is set… I don’t think you need to check if date_edited exists so you have a much simpler query:

$args = array(
    'post_type' => 'page',
    'meta_query' => array(
        'key' => 'footnote',
        'value'   => '',
        'compare' => 'NOT EQUALS'
    ),
    'post_status' => 'publish',
    'posts_per_page' => 10,
    'no_found_rows' => true,
    'orderby' => 'meta_value_num',
    'meta_key' => 'date_edited',
    'order' => 'DESC'
);

Hope this helps… and that your fever has broken? 🙂