var legend = new Legend({
        map: map,
        layer:D_ml.visibleLayers
    }, "legendDiv");
    legend.startup();

[![enter image description here][1]][1]C_ml = new ArcGISDynamicMapServiceLayer("http://imcltpdarcp01.duke-energy.com/ArcGIS/rest/services/TEST_DEC_DEI/MapServer");//,{opacity: 0.6};

    D_ml = new ArcGISDynamicMapServiceLayer("http://imcltpdarcp01.duke-energy.com/ArcGIS/rest/services/TEST_DEC_DEI/MapServer",{opacity: 0.75});

    map.addLayers([C_ml,D_ml]);

    C_ml.setVisibleLayers([2,3]);
    D_ml.setVisibleLayers([0,1]);