Sorry, I've slightly not understood what you want. I've created 2 widget in designer (Form1 and Form2) and placed they to QWidgetStack. Also, I've created an joint widget (Form1 + Form2) and placed it to QWidgetStack also:
Qt Code: 
  1. int main( int argc, char ** argv )
  2. {
  3.     QApplication a( argc, argv );
  4.     FormMain w;
  5.  
  6.     w.widgetStack1->addWidget(new Form1(&w));
  7.     w.widgetStack1->addWidget(new Form2(&w));
  8.  
  9.     QWidget * MultiView = new QWidget(&w);
  10.     QHBoxLayout * lo = new QHBoxLayout(MultiView);
  11.     lo->addWidget(new Form1(MultiView));
  12.     lo->addWidget(new Form2(MultiView));
  13.  
  14.     w.widgetStack1->addWidget(MultiView);
  15.  
  16.     w.show();
  17.     w.widgetStack1->raiseWidget(MultiView);
  18.  
  19.     a.connect( &a, SIGNAL( lastWindowClosed() ), &a, SLOT( quit() ) );
  20.     return a.exec();
  21. }
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It does work ok. Is it that which necessary for you?