GroupTileLayer is used to display a group of tile layers. Its performance is better than add TileLayers seperately and it can prevent the warning:

"WARNING: Too many active WebGL contexts. Oldest context will be lost" on chrome

index.html
<!DOCTYPE html>
<html>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>TileLayer and Geo-Projections - Add GroupTileLayer</title>
  <style type="text/css">
    html,body{margin:0px;height:100%;width:100%}
    .container{width:100%;height:100%}
  </style>
  <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/maptalks/dist/maptalks.css">
  <script type="text/javascript" src="https://cdn.jsdelivr.net/npm/maptalks/dist/maptalks.min.js"></script>
  <body>
    <div id="map" class="container"></div>

    <script>
      var map = new maptalks.Map('map', {
        center: [-0.113049, 51.498568],
        zoom: 6,
        pitch: 40,
        attribution: {
            content: ' &copy; carto'
        },
        // add 2 TileLayers with a GroupTileLayer
        baseLayer: new maptalks.GroupTileLayer('base', [
            new maptalks.TileLayer('tile2', {
                urlTemplate: 'https://{s}.basemaps.cartocdn.com/light_nolabels/{z}/{x}/{y}.png',
                subdomains: ['a', 'b', 'c', 'd']
            }),

            new maptalks.TileLayer('boudaries', {
                'urlTemplate': 'https://{s}.basemaps.cartocdn.com/dark_only_labels/{z}/{x}/{y}.png',
                'subdomains': ['a', 'b', 'c', 'd']
            })
        ])
      });


    </script>
  </body>
</html>
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