public static class IntAggregator
{
    public static void Transmit(int data)
    {
        if (OnDataTransmitted != null)
        {
            OnDataTransmitted(data);
        }
    }

    public static Action<int> OnDataTransmitted;
}

public class ModifyUsersViewModel : INotifyPropertyChanged
{
    private void Send_Data()
    {
        IntAggregator.Transmit(2);
    }
}

public class MainWindowViewModel : INotifyPropertyChanged
{
    private int _Tab_SelectedIndex = 0;
    public int Tab_SelectedIndex
    {
        get
        {
            return _Tab_SelectedIndex;
        }
        set
        {
            _Tab_SelectedIndex = value;
            OnPropertyChanged(new PropertyChangedEventArgs("Tab_SelectedIndex"));
        }
    }

    public MainWindowViewModel()
    {
        IntAggregator.OnDataTransmitted += OnDataReceived;
    }

    private void OnDataReceived(int data)
    {
        Tab_SelectedIndex = 2;
    }
}