Creating three instances of the CMS application (two of them i a dialog window).

main.js

require([ './EntryPoint', 'dijit/Dialog', 'dojo/domReady!' ],
function (EntryPoint, Dialog) {
  app.entryPoint = new EntryPoint().placeAt(document.body);
  app.entryPoint.startup();
  
  launchInDialog(new EntryPoint());
  launchInDialog(new EntryPoint());

  function launchInDialog(application) {
    var placeholder = document.createElement("div");
    placeholder.style.width = "800px";
    placeholder.style.height = "500px";
  
    var dialog = new Dialog({ content: placeholder });
    dialog.startup();
    dialog.show();
    
    application.placeAt(placeholder);
    application.startup();
    
    return dialog;
  }
});