如何在Xt/Motif中创建两个顶层窗口？

#include <Xm/Xm.h>

int main(int argc, char **argv) {
  XtAppContext app;
  Widget topLevel, mainWindow;

  topLevel = XtVaAppInitialize(&app, "MyApp", NULL, 0, &argc, argv, NULL, NULL);
  mainWindow = XtVaCreatePopupShell("MainWindow", topLevelShellWidgetClass, topLevel,
                                    XmNwidth, 500, XmNheight, 400, NULL);

  XtRealizeWidget(topLevel);
  XtAppMainLoop(app);

  return 0;
}

#include <Xm/Xm.h>

void createSecondWindow(Widget parentWindow) {
  Widget secondWindow;

  secondWindow = XtVaCreatePopupShell("SecondWindow", topLevelShellWidgetClass, parentWindow,
                                      XmNwidth, 300, XmNheight, 200, NULL);

  XtRealizeWidget(secondWindow);
}

int main(int argc, char **argv) {
  XtAppContext app;
  Widget topLevel, mainWindow;

  topLevel = XtVaAppInitialize(&app, "MyApp", NULL, 0, &argc, argv, NULL, NULL);
  mainWindow = XtVaCreatePopupShell("MainWindow", topLevelShellWidgetClass, topLevel,
                                    XmNwidth, 500, XmNheight, 400, NULL);

  createSecondWindow(mainWindow);

  XtRealizeWidget(topLevel);
  XtAppMainLoop(app);

  return 0;
}