DevExpress GridColumn strange proportional sizing

I wish this helpful for you From my experience you can't set the default value of the repository item. The only way to do that is to set the value of your datasource properties to the value you want in the comboBox. Or if its an unbound column to use the CustomUnboundColumnData event to set the value. So that you are basically setting the cell value which happens to match what you want in the repositoryItemComboBox.
For example:

List<whatever> list = new List<whatever>();

foreach (whatever item in list) {
    item.property = repository.Items(0);
}

hop of those help? Here is another, alternative answer that you can modify into your needs.
In this example, you can enter the width of your grid in the textbox in the first column. Or you can expand or decrease the width with the button. Just for illustration. You may have to change this for your purpose.

public partial class MainWindow : Window, INotifyPropertyChanged
{
    public static readonly DependencyProperty GridWidthProperty = DependencyProperty.Register("GridWidth", typeof(Double), typeof(MainWindow), new UIPropertyMetadata(300d, OnGridWidthPropertyChanged));
    public Double GridWidth
    {
        get { return (Double)GetValue(GridWidthProperty); }
        set
        {
            SetValue(GridWidthProperty, value);
            NotifyPropertyChanged("GridWidth");
        }
    }

    public static readonly DependencyProperty ColumnWidthProperty = DependencyProperty.Register("ColumnWidth", typeof(String), typeof(MainWindow), new UIPropertyMetadata("100", OnColumnWidthPropertyChanged));
    public String ColumnWidth
    {
        get { return (String)GetValue(ColumnWidthProperty); }
        set
        {
            SetValue(ColumnWidthProperty, value);
            NotifyPropertyChanged("ColumnWidth");
        }
    }

    private static void OnGridWidthPropertyChanged(DependencyObject sender, DependencyPropertyChangedEventArgs args)
    {
        MainWindow ctl = sender as MainWindow;

        ctl.doGridWidthChanged();
        ctl = null;
    }

    private static void OnColumnWidthPropertyChanged(DependencyObject sender, DependencyPropertyChangedEventArgs args)
    {
        MainWindow ctl = sender as MainWindow;

        ctl.doColumnWidthChanged();
        ctl = null;
    }

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

    private void button_Click(object sender, RoutedEventArgs e)
    {
        if (sender == button1)
            this.GridWidth += 50;
        else if (sender == button2)
            this.GridWidth -= 50;
    }

    private void doGridWidthChanged()
    {
        if (Double.IsNaN(this.GridWidth))
            return;

        this.ColumnWidth = Math.Round((this.GridWidth / 3), 2).ToString();
    }

    private void doColumnWidthChanged()
    {
        Double columnwidthval = Double.NaN;

        if (!String.IsNullOrEmpty(this.ColumnWidth) && Double.TryParse(this.ColumnWidth, out columnwidthval))
            this.GridWidth = columnwidthval * 3;
        else
            this.ColumnWidth = Math.Round((this.GridWidth / 3), 2).ToString();
    }

    public event PropertyChangedEventHandler PropertyChanged;
    public void NotifyPropertyChanged(String PropertyName)
    {
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs(PropertyName));
    }
}

<Window x:Class="WpfApplication3.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:WpfApplication3"
        Title="MainWindow" Height="600" Width="800">
    <Grid>
        <Grid Margin="0,60,0,0"
              Width="{Binding Path=GridWidth}">
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="1*" />
                <ColumnDefinition Width="2*" />
            </Grid.ColumnDefinitions>
            <Border Grid.Column="0" Background="GhostWhite" />
            <Border Grid.Column="1" Background="AliceBlue" />
            <Border Grid.ColumnSpan="2" BorderBrush="DimGray" BorderThickness="1" />
            <StackPanel Grid.Column="0" Orientation="Vertical" Margin="3">
                <TextBlock Text="Single" />
                <TextBox Text="{Binding Path=ColumnWidth, Mode=TwoWay}" />
            </StackPanel>
            <StackPanel Grid.Column="1" Orientation="Vertical" Margin="3">
                <TextBlock Text="Double" />
            </StackPanel>
        </Grid>
        <Button Content="Increase" Height="34" HorizontalAlignment="Left" Margin="19,12,0,0" Name="button1" VerticalAlignment="Top" Width="90" Click="button_Click" />
        <Button Content="Decrease" Height="34" HorizontalAlignment="Left" Margin="120,12,0,0" Name="button2" VerticalAlignment="Top" Width="90" Click="button_Click" />
    </Grid>
</Window>

around this issue I managed to find the solution. The problem was the datatrigger binding instead of Data.SaveStatus should be RowData.Row.SaveStatus:

 <Style.Triggers>
                <DataTrigger Binding="{Binding Path=RowData.Row.SaveStatus, UpdateSourceTrigger=PropertyChanged}" Value="{x:Static enums:SaveState.DoneSuccesfuly}">
                    <Setter Property="Source" Value="..\Icons\StatusOk.png"></Setter>
                </DataTrigger>
                <DataTrigger Binding="{Binding Path=RowData.Row.SaveStatus, UpdateSourceTrigger=PropertyChanged}"  Value="{x:Static enums:SaveState.DoneUnsuccesfuly}">
                    <Setter Property="Source" Value="..\Icons\StatusError.png"></Setter>
                </DataTrigger>
            </Style.Triggers>

This might help you Follow below steps, it will work fine.
Place tablelayoutpanel control in tabcontrol. Set rows and columns for tablelayoutpanel using the "Edit Rows/Columns" If you have 2 datagridviews only in this tabcontrol, then set row count 1,column count 1. Set column1 with Percentage 100 ,row1 with 50%, row2 with 50% Set tablelayoutpanel dock property to fill. drag and drop datagridview1 to tablelayoutpanel row1 and set the dock property to fill. drag and drop datagridview2 to tablelayoutpanel row2 and set the dock property to fill.