How do i hightlight one textbox

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Silverlight

I have two TextBox in a cell of DataGrid in silverlight,when the cell put into edit mode ,both of them highlight,but i just want one.For example,when i click button1 ,the first TextBox highlight.I click button2 another hightlight.How to do it?This is my code

<my:DataGridTemplateColumn.CellEditingTemplate>
                                        <DataTemplate>
                                            <Grid x:Name="Root">
                                                <Grid>
                                                    <Grid.RowDefinitions>
                                                        <RowDefinition Height="25" />
                                                        <RowDefinition Height="1" />
                                                        <RowDefinition Height="25" />
                                                    </Grid.RowDefinitions>
                                                    <Grid.ColumnDefinitions>
                                                        <ColumnDefinition Width="120"/>
                                                    </Grid.ColumnDefinitions>
                                                    <!-- Row 0 -->
                                                    <TextBox Background="#FFCCFAFB" Text="{Binding StartNO,Mode=TwoWay}"/>
                                                    <!-- Row 1 -->
                                                    <Line Stroke="#FF8CB2C6" X1="1" X2="84" Y1="1" StrokeDashArray="2" Grid.Row="1" Y2="1" />
                                                    <!--<Rectangle Fill="#FFC9CACA" VerticalAlignment="Stretch" Height="1" Visibility="Visible" Grid.Row="1"/>-->
                                                    <!-- Row 2 -->
                                                    <TextBox Background="#FFCCFAFB" Text="{Binding EndNO,Mode=TwoWay}" Grid.Row="2"/>
                                                </Grid>
                                            </Grid>
                                        </DataTemplate>
                                    </my:DataGridTemplateColumn.CellEditingTemplate>