<Grid>
  <Grid.ColumnDefinitions>
    <ColumnDefinition Width="{Binding Col1, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
    <ColumnDefinition Width="Auto"/>
    <ColumnDefinition Width="*"/>
  </Grid.ColumnDefinitions>
  <TextBox
    Text="Column1"
    VerticalAlignment="Stretch"/>
  <GridSplitter
    Grid.Column="1"
    Width="2"
    Margin="2"
    Background="Black"
    HorizontalAlignment="Center"
    VerticalAlignment="Stretch"
    ResizeDirection="Columns"
    ResizeBehavior="PreviousAndNext"/>
  <TextBox
    Grid.Column="2"
    Text="Column2"
    VerticalAlignment="Stretch"/>
</Grid>

public partial class MainWindow: INotifyPropertyChanged
{

  [DllImport("kernel32.dll",
    EntryPoint = "AllocConsole",
    SetLastError = true,
    CharSet = CharSet.Auto,
    CallingConvention = CallingConvention.StdCall)]
  public static extern int AllocConsole();

  public MainWindow()
  {
    InitializeComponent();
    AllocConsole();
    DataContext = this;
  }

  GridLength _col1 = new GridLength(400);
  public GridLength Col1
  {
    get { return _col1; }
    set
    {
      _col1 = value;
      RaisePropertyChanged("Col1");
      Console.WriteLine("Col1 setter");
    }
  }

  public event PropertyChangedEventHandler PropertyChanged;
  private void RaisePropertyChanged(string propertyName)
  {
    if (PropertyChanged == null) return;
    PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
  }
}

<Grid>
  <Grid.RowDefinitions>
    <RowDefinition />
    <RowDefinition Height="Auto"/>
  </Grid.RowDefinitions>
  <Grid>
    <Grid.ColumnDefinitions>
      <ColumnDefinition Width="{Binding Col1, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
      <ColumnDefinition Width="Auto"/>
      <ColumnDefinition Width="*"/>
    </Grid.ColumnDefinitions>
    <TextBox
      Text="Column1"
      VerticalAlignment="Stretch"/>
    <GridSplitter
      Grid.Column="1"
      Width="2"
      Margin="2"
      Background="Black"
      HorizontalAlignment="Center"
      VerticalAlignment="Stretch"
      ResizeDirection="Columns"
      ResizeBehavior="PreviousAndNext"/>
    <TextBox
      Grid.Column="2"
      Text="Column2"
      VerticalAlignment="Stretch"/>
  </Grid>
  <Button
    Grid.Row="2"
    Content="test"
    HorizontalAlignment="Center"
    VerticalAlignment="Center"
    Margin="10"
    Command="{Binding TestCommand}"/>
</Grid>

public partial class MainWindow: INotifyPropertyChanged
{
  [DllImport("kernel32.dll",
    EntryPoint = "AllocConsole",
    SetLastError = true,
    CharSet = CharSet.Auto,
    CallingConvention = CallingConvention.StdCall)]
  public static extern int AllocConsole();

  public MainWindow()
  {
    InitializeComponent();
    AllocConsole();
    TestCommand = new TestCommand(()=>Col1 = new GridLength(Col1.Value+50));
    DataContext = this;
  }

  public TestCommand TestCommand {get; private set;}

  GridLength _col1 = new GridLength(400);

  public GridLength Col1
  {
    get { return _col1; }
    set
    {
      _col1 = value;
      RaisePropertyChanged("Col1");
      Console.WriteLine("Col1 setter");
    }
  }

  public event PropertyChangedEventHandler PropertyChanged;
  private void RaisePropertyChanged(string propertyName)
  {
    if (PropertyChanged == null) return;
    PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
  }
}

public class TestCommand: ICommand
{
  readonly Action _action;
  public TestCommand(Action action)
  {
    _action = action;
  }
  public void Execute(object parameter)
  {
    _action();
  }

  public bool CanExecute(object parameter)
  {
    return true;
  }

  public event EventHandler CanExecuteChanged;
}