VRT file to write both 'the_geom' and 'the_geom_webmercator' columns

both_geoms.vrt

<?xml version="1.0"?>
<OGRVRTDataSource>
  <OGRVRTWarpedLayer>
    <OGRVRTLayer name="cartogeom">
      <SrcDataSource>cartogeom.csv</SrcDataSource>
      <GeometryField name="the_geom" encoding="PointFromColumns" x="longitude" y="latitude">
        <GeometryType>wkbPoint</GeometryType>
        <SRS>EPSG:4326</SRS>
      </GeometryField>
      <GeometryField name="the_geom_webmercator" encoding="PointFromColumns" x="longitude" y="latitude">
        <GeometryType>wkbPoint</GeometryType>
        <SRS>EPSG:4326</SRS>
      </GeometryField>
      <Field name="name" />
      <Field name="desc" />
    </OGRVRTLayer>
    <WarpedGeomFieldName>the_geom_webmercator</WarpedGeomFieldName>
    <TargetSRS>EPSG:3857</TargetSRS>
  </OGRVRTWarpedLayer>
</OGRVRTDataSource>