Ustawienie różnych DataContext dla poszczególnych atrybutów kontrolki ComboBox

<ComboBox x:Name="SelectedElement"  ItemsSource="{Binding ListElementsBook}"  SelectedIndex="{Binding Index}" />

class ListsViewModel : BaseViewModel
    {        
        private ObservableCollection<string> _listElementsBook;
        public ObservableCollection<string> ListElementsBook
        {
            get { return _listElementsBook; }
            set{_listElementsBook = value; OnPropertyChange();}
        }
     }

class SomeViewModel : BaseViewModel
    {        
        private int _index;
        public int Index
        {
            get { return _index; }
            set{ _index = value; OnPropertyChange();}
        }
     }