Currently, the QmitkStdMultiWidget has 4 members. One for each renderwindow. The QmitkStdMultiWidgetEditor, however, uses a QHash with a QString to identify the different render windows. This should be ported to the widget itself:

QHash<QString, QmitkRenderWindow*> m_RenderWindows;

Afterwards, it should also be used in the QmitkStdMultiWidgetEditorPreferencePage to fill the combo box for selecting the respective widget.