LeftRightLayoutControl.cs

public class LeftRightLayoutControl : StackPanel
{
    private Grid _grid;
    private StackPanel _left;
    private StackPanel _right;

    public LeftRightLayoutControl()
    {
        _grid = new Grid();
        _grid.RowDefinitions.Add(new RowDefinition { Height = new GridLength(1, GridUnitType.Star) });
        _grid.ColumnDefinitions.Add(new ColumnDefinition { Width = new GridLength(1, GridUnitType.Star) });
        _grid.ColumnDefinitions.Add(new ColumnDefinition { Width = new GridLength(1, GridUnitType.Star) });

        _left = new StackPanel();
        Grid.SetRow(_left, 0);
        Grid.SetColumn(_left, 0);
        _grid.Children.Add(_left);

        _right = new StackPanel();
        Grid.SetRow(_right, 0);
        Grid.SetColumn(_right, 1);
        _grid.Children.Add(_right);

        Children.Add(_grid);
    }
}