$combined = array();
if ( !empty( $feed_posts ) ) {
    foreach($feed_posts as $post)
    {
        $combined[$post['date_added']][] = $post; // unix timestamp in order to sort
    }
}

if ( !empty( $hardnote_photos ) ) {
    foreach($hardnote_photos as $photo)
    {
        $combined[$photo['date_created']][] = $photo; // unix timestamp in order to sort
    }
}

krsort($combined); // sort the keys