Using two layers data and showing those values in the infowindow

<!DOCTYPE html>
<html>
  <head>
    <title>Custom infowindow example | CartoDB.js</title>
    <meta name="viewport" content="initial-scale=1.0, user-scalable=no" />
    <meta http-equiv="content-type" content="text/html; charset=UTF-8"/>
    <link rel="shortcut icon" href="https://cartodb.com/assets/favicon.ico" />
    
    <style>html, body, #map { height: 100%; padding: 0; margin: 0 }</style>
    <link rel="stylesheet" href="https://libs.cartocdn.com/cartodb.js/v3/themes/css/cartodb.css" />
  </head>
  <body>
    <div id="map"></div>

<script src="https://libs.cartocdn.com/cartodb.js/v3/cartodb.js"></script>
    
    <script type="text/javascript">
      var layer;

function main() {

var map = L.map('map', { 
          zoomControl: false,
          center: [0, 0],
          zoom: 3
        });

// add a nice baselayer from Stamen 
        L.tileLayer('https://a.tiles.mapbox.com/v3/examples.map-20v6611k/{z}/{x}/{y}.png', {
          attribution: 'MapBox'
        }).addTo(map);

cartodb.createLayer(map, {
          user_name: 'documentation',
          type: 'cartodb',
          sublayers: [{
            sql: "select european_countries_e.cartodb_id, world_borders.subregion, world_borders.un, european_countries_e.area, european_countries_e.the_geom, european_countries_e.the_geom_webmercator from european_countries_e, world_borders WHERE european_countries_e.iso_2_code = world_borders.iso2",
            layer_name: "european_countries_e",
            cartocss: "#european_countries_e{ line-color: #FFF; line-opacity: 1; line-width: 1; polygon-opacity: 0.8; } #european_countries_e [ area <= 1638094.0] { polygon-fill: #B10026; } #european_countries_e [ area <= 34895.0] { polygon-fill: #E31A1C; } #european_countries_e [ area <= 24193.0] { polygon-fill: #FC4E2A; } #european_countries_e [ area <= 10025.0] { polygon-fill: #FD8D3C; } #european_countries_e [ area <= 6889.0] { polygon-fill: #FEB24C; } #european_countries_e [ area <= 4808.0] { polygon-fill: #FED976; } #european_countries_e [ area <= 3288.0] { polygon-fill: #FFFFB2; }",
            interactivity: "cartodb_id"
          }]
        })
          .addTo(map)
          .on('done', function(lyr) {

cdb.vis.Vis.addInfowindow(map, lyr.getSubLayer(0), ['cartodb_id', 'subregion', 'un'])

}).on('error', function() {
            console.log("some error occurred");
          });
      }

window.onload = main;
      
    </script>
  </body>
</html>

Modified http://libs.cartocdn.com/cartodb.js/v3/cartodb.js to a secure url

https://libs.cartocdn.com/cartodb.js/v3/cartodb.js