Cluster multiple feature server

I want to cluster different servers at a time, what I mean is this I have this feature servers: server1=http://ift.tt/1BBGrlJ

server2=http://ift.tt/1DC3bPc

When using L.esri.clusteredFeatureLayer('server1', { radius: 120000000000, useCORS:0, })addTo(map); for both of them they are clustered separately. I have tried using something like

L.esri.Layers.featureLayer('http://server1/0').on('ready', function(e) {
// The clusterGroup gets each marker in the group added to it
// once loaded, and then is added to the map
var clusterGroup = new L.MarkerClusterGroup();
eachLayer(function(layer) {
    clusterGroup.addLayer(layer);
});

});

to add markers to a clustergroup and then add the markers from server2, but cant get it working any help?