How to sort combind field in datagrid.

I use the code below to combine

<sdk:DataGridTemplateColumn Header="MergeColumn" Width="90">
<sdk:DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<StackPanel Orientation="Horizontal" Margin="3,4,0,0">
<TextBlock Text="{Binding FieldA}" />
<TextBlock Text="{Binding FieldB}" />
</StackPanel>
</DataTemplate>
</sdk:DataGridTemplateColumn.CellTemplate>
</sdk:DataGridTemplateColumn>