var viewer = new Cesium.Viewer('cesiumContainer');

var dataSource1 = new Cesium.KmlDataSource();
dataSource1.load('../../SampleData/kml/facilities/facilities.kml');

var dataSource2 = new Cesium.KmlDataSource();
dataSource2.load('../../SampleData/kml/gdpPerCapita2008.kmz');

Sandcastle.addToolbarButton('Add DataSource 1', function() {
    viewer.dataSources.add(dataSource1);
});

Sandcastle.addToolbarButton('Remove DataSource 1', function() {
    viewer.dataSources.remove(dataSource1);
});

Sandcastle.addToolbarButton('Add DataSource 2', function() {
    viewer.dataSources.add(dataSource2);
});

Sandcastle.addToolbarButton('Remove DataSource 2', function() {
    viewer.dataSources.remove(dataSource2);
});

If you are not familiat with Promises, I recommend reading this article: http://www.html5rocks.com/en/tutorials/es6/promises/ Cesium specifically uses the `when` promise library that is mentioned in the article.

 
var viewer = new Cesium.Viewer('cesiumContainer');

var promise1 = Cesium.KmlDataSource.load('../../SampleData/kml/facilities/facilities.kml');
var promise2 = Cesium.KmlDataSource.load('../../SampleData/kml/gdpPerCapita2008.kmz');

Cesium.when(promise1, function(dataSource1){
    Sandcastle.addToolbarButton('Add DataSource 1', function() {
        viewer.dataSources.add(dataSource1);
    });

    Sandcastle.addToolbarButton('Remove DataSource 1', function() {
        viewer.dataSources.remove(dataSource1);
    });
});

Cesium.when(promise2, function(dataSource2){
    Sandcastle.addToolbarButton('Add DataSource 2', function() {
        viewer.dataSources.add(dataSource2);
    });

    Sandcastle.addToolbarButton('Remove DataSource 2', function() {
        viewer.dataSources.remove(dataSource2);
    });
});