How do I create my own nested meta_query using posts_where / posts_join?

Here is a rough outline of how you’d go about making a UNION work with WP_Query.

add_filter(
  'posts_request',
  function ($clauses) {

    $clauses = str_replace('SQL_CALC_FOUND_ROWS','',$clauses,$scfr);

    $scfr = (0 < $scfr) : 'SQL_CALC_FOUND_ROWS' : '';

    $clause2 = $clauses; // manipulate this

    return "SELECT {$scfr} u.* FROM (({$clauses}) UNION ({$clause2})) as u";
  },
  1,
  2
);

Set up your WP_Query arguments to generate the first half of the UNION then manipulate that to create the whole query. Something like this should be close.

add_action('pre_get_posts', 'my_search_price');
function my_search_price( $query ) {
    if ($query->get('maxprice') != "" && $query->get('minprice') != "" && $query->is_main_query()) {

        $maxprice = intval($query->get('maxprice'));
        $minprice = intval($query->get('minprice'));

        $meta = array();

        $meta[] =
          array (
              'key' => 'price',
              'value' => array($maxprice,$minprice),
              'compare' => 'between',
          );

        $meta[] = 
          array (
              'key' => 'app_updated',
              'value' => time()-(60*60*24*14),
              'compare' => '>',
              'type' => 'numeric'
          );

        $query->set('meta_query', $meta);
    }
}

add_filter(
  'posts_request',
  function ($clauses) {

    $clauses = str_replace('SQL_CALC_FOUND_ROWS','',$clauses,$scfr);

    $scfr = (0 < $scfr) ? 'SQL_CALC_FOUND_ROWS' : '';

    $clause2 = str_replace('price','price_used',$clauses); // manipulate this
    $clause2 = str_replace('app_updated','price_used_updated',$clauses);

    return "SELECT {$scfr} u.* FROM (({$clauses}) UNION ({$clause2})) as u";
  },
  1,
  2
);

You will need to add logic to the post_requests filter so that it does not run when you don’t want it to.