X Window radio button in submenu question

I have a question, I have a menu and a submenu. Both
use radio buttons. How can I make it so that a menu
and a submenu act as one group of radio buttons?

Example: if I select a button in a submenu, the
previously selected button in a main menu should get
unselected. The same if I select a button in the main
menu, the selected button in a submenu should be
unselected.
I create 2 pull down menus using XmCreatePulldownMenu,
and
XtVaSetValues(Name,
XmNradioBehavior,True,
XmNentryCallback,List,
NULL);
to set arguments for both menus.
List is
XtCallbackRec List[2];
List[0].callback = _DTypeChanged;
List[0].closure = (XtPointer)DType;
List[1].callback = 0;
List[1].closure = 0;

The first element of the main menu is created using

Widget w = XtVaCreateManagedWidget("Name1",
        xmCascadeButtonWidgetClass, parent,
        XmNsubMenuId,pullDownP,
        XmNmnemonic, 'F',
        NULL);

I can switch between radio buttons in a submenu or a
menu, but not between menu and a submenu. 
Thank you
Anna

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