struct MyDlg : TopWindow {
    WithSubDlg1Layout<ParentCtrl> subdlg1;
    WithSubDlg2Layout<ParentCtrl> subdlg2;

    void Show1() { subdlg1.Show(); subdlg2.Hide(); }
    void Show2() { subdlg1.Hide(); subdlg2.Show(); }

    Button b1, b2; // Placing buttons is omitted

    typedef MyDlg CLASSNAME;

    MyDlg() {
       CtrlLayout(subdlg1);
       CtrlLayout(subdlg2);
       Add(subdlg1.SizePos());
       Add(subdlg2.SizePos());
       b1 <<= THISBACK(Show1);
       b2 <<= THISBACK(Show2);
       Show1();
    }
};