Get all info feature of layers within polygon from another layer

Question

I found this example here - (question details for example). It allows to get all the features in the drawn polygon. It's almost what i needed, but instead of drawing, I want when clicking on the feature of the upper layer(layerA) to get all the feature info of layer B within polygon of layerA.

Here is two layers that I have:

Layer A

var layerA = new ol.layer.Image({
    source: new ol.source.ImageWMS({
        url: 'https://xxxxxxx/geoserver/xxx/wms',
        params: {LAYERS: 'xxx:xxxa', 'TILED':true, 'format':'image/png8'},
        serverType: 'geoserver',
        ratio: 1,
    }),
    minZoom: 10,
    visible: true,
})
map.addLayer(layerA);

Layer B

var layerB = new ol.layer.Image({
    source: new ol.source.ImageWMS({
        url: 'https://xxxxxxx/geoserver/xxx/wms',
        params: {LAYERS: 'xxx:xxxb', 'TILED':true, 'format':'image/png8'},
        serverType: 'geoserver',
        ratio: 1,
    }),
    minZoom: 10,
    visible: true,
})
map.addLayer(layerB);

By this way I can get feature info but only at click position:

function showInfo(coord, resol) {
  var content = document.getElementById('info');
  var url = layerA.getSource().getFeatureInfoUrl(
    coord,
    resol,
    'EPSG:3857',
    {
      'INFO_FORMAT': 'application/json',
      'propertyName':'name,id'
    },
  );
  if(url) {
    $.getJSON(url, function(data) {
      var feature = data.features[0];
      var props = feature.properties;
      content.innerHTML = '<h5>name : </h5><p>' + props.name + '</p>' + ...
    });
    }
  else {
    content.innerHTML = ''
  }
}

map.on('click', function(evt) {
  showInfo(evt.coordinate, map.getView().getResolution());
})