How to concatenate two separate colums into one?

Seems like you are looking for a pure SQL solutions for this. You can achieve this by running the following query:

UPDATE wp_posts 
SET latitude_longitude = CONCAT(latitude, ", " , longitude);

That will work for you, just double check the name of the columns: latitude_longitude, latitude and longitude.

Update

If you want to run that SQL query each time a post is saved you can use a hook as shown below:

function update_post_location( $post_id ) {

    // If this is just a revision, do not update.
    if ( ! wp_is_post_revision( $post_id ) )
        return;

    $query = 'UPDATE wp_posts ' . 
             'SET latitude_longitude = CONCAT(latitude, ", " , longitude) ' .
             'WHERE id = ' . $post_id;

    $wpdb->query( $query )
}
add_action( 'save_post', 'update_post_location' );