How to add multiple layers to OpenLayers from a single Geoserver WMS?

Question

I'm trying to add multiple layers from a single Geoserver source to my Openlayers map. I can get each layer to appear by changing a single line of code. but when I attempt to have both layers on at once, I get a blank map. How do I add both? One is a raster, the other is a vector.

I've attempted via having the additional layer in a new variable and using map.addlayer. This didn't break the map, but it still didn't load the additional layer. I tried by just adding the additional layer into the layer variable, broke the map. I also tried by adding it to the params. So it was

params:{'LAYERS': 'Test:TestImagery', 'Test:Vector'},

This broke the map. How do I make this work?

Code:

<script>
  var layers = [
    new ol.layer.Tile({
      source: new ol.source.OSM()
    }),

    new ol.layer.Image({
      extent: [-20000000, 1000000, -5000000, 9000000],
      source: new ol.source.ImageWMS({
        url: 'http://localhost/geoserver/Test/wms',
        params: {'LAYERS': 'Test:TestImagery'},
        ratio: 1,
        serverType: 'geoserver'
      })
    })

  ];
  var map = new ol.Map({
    layers: layers,
    target: 'map',
    view: new ol.View({
      center: [0,0],
      zoom: 14
    })
  });

</script>

Answer 1

Credit to nmtoken.

for having multiple layers from the same WMS, code should read:

params:{'LAYERS': 'Test:TestImagery,Test:Vector'},