Feed the ListBox the two pieces of data separately, and use a data template. Here's how.
First, create a little class to represent each item you want to insert:
public class WordPair {
public string First { get; set; }
public string Second { get; set; }
}
(You probably already have a suitable class and/or collection in your application -- I assume those pairs of strings are coming from somewhere!)
Second, set your ListBox.ItemsSource to a collection of these things:
listBox.ItemsSource = new List<WordPair> {
new WordPair { First = "ITEM A", Second = "ITEM B" },
new WordPair { First = "ITEM X", Second = "ITEM Y" },
};
Again, this collection may already exist in your app.
Third, create a DataTemplate specifying the desired layout, and assign it to your ListBox.ItemTemplate:
<!-- in your Window.Resources section -->
<DataTemplate x:Key="AlignedPairs">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*" />
<ColumnDefinition Width="Auto" />
<ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>
<TextBlock Text="{Binding First}" Grid.Column="0" />
<TextBlock Text="->" TextAlignment="Center" Grid.Column="1" />
<TextBlock Text="{Binding Second}" TextAlignment="Right" Grid.Column="2" />
</Grid>
</DataTemplate>
<ListBox Name="listBox" ItemTemplate="{StaticResource AlignedPairs}">
<ListBox.ItemContainerStyle>
<Style TargetType="ListBoxItem">
<Setter Property="HorizontalContentAlignment" Value="Stretch" />
</Style>
</ListBox.ItemContainerStyle>
</ListBox>
(I've guessed at the exact alignment you want for the items, but you can obviously tweak it.)
Note that you also need to set the HorizontalContentAlignment of the ListBoxItems to Stretch using ListBox.ItemContainerStyle. Otherwise each ListBoxItem will take up only the space it needs, resulting in all the Grid columns being minimal size and looking like a straight concatenation. Stretch makes each ListBoxItem fill the full width so the Grid columns are forced to grow accordingly.