How to sort a WP_Query by a custom field AND ALSO filter by a different custom field

to sort the posts by a meta field you need to have this field in meta_query and in orderby like this

$query_args = array(
  'post_type' => 'MY_POSTTYPE_NAME',
  'post_per_page' => -1,
  'meta_query' => array( 
      'show_post_query' => array(
          'key' => 'SHOW_POST',
          'value' => '1'
      ),
      'MY_FIELD_NAME__order_by' => array(
          'key' => 'MY_FIELD_NAME',
          'type' => 'NUMERIC',
          'compare' => 'NUMERIC',
      )
  ),
  'orderby' => array( 'MY_FIELD_NAME__order_by' => 'ASC' ),
);
$query = new WP_Query( $query_args );

Note the MY_FIELD_NAME__order_by does not have “value” set so it will not be used for searching but will be (let’s say) named so you can use it as column name when sorting.