//silhouetteGreen.selected = [];
silhouetteGreen.selected.push(pickedFeature);
You're in complete control of that logic in your application. Notice that this example clears all previously selected buildings on click. You can change this logic, so comment out line 93:
//silhouetteGreen.selected = [];
And then change line 114 to add onto the array instead of replacing it:
silhouetteGreen.selected.push(pickedFeature);
And that will allow you to select multiple buildings. You can also change the UI to loop over all the selected buildings to display their names/properties instead of just the last selected one. Hope this helps! What kind of application are you working on?