public MainWindowViewModel(ILeftView firstViewModel, IRightView secondviewModel, IWindowManager _windowManager) // etc, for each child ViewModel
 {
     LeftView = firstViewModel;
     RightView = secondviewModel;
     windowManager = _windowManager;
 }
 public MainWindowViewModel(ILeftView firstViewModel, IRightView secondviewModel, IWindowManager _windowManager) // etc, for each child ViewModel
 {
     LeftView      = firstViewModel;
     RightView     = secondviewModel;
     windowManager = _windowManager;
 }