XElement unable to perform twoway binding - Readonly

I'm using an Ienumerable<XElement> in a listbox to which I get back the list of elements and display the name attribute. On Selection of those elements the entire element is then displayed in a list box.

However when setting the mode to twoway I am being told that I cannot use two way with a readonly property, despite the set command being unique.

I think I'm having a fundamental misunderstanding and was hoping you could clue me in.

Steps is my collection of all the CS

 public IEnumerable<XElement> Steps
    {
        get
        {
            if (_XDoc == null) return null; 
            else 
                return (_XDoc.Descendants().Where(x => x.Name == ns + "Core")); 

        }
        set
        {
            XElement xNode = Steps.Elements().Except(value).First();
            XElement replaceNode = value.Elements().Except(Steps).First();
            _XDoc.ToString().Replace(xNode.ToString(), value.ToString());
        }
       }

XAML

<ListView HorizontalAlignment="Stretch" Name="listView1" VerticalAlignment="Top" Grid.Column="0" Grid.Row="1"
       DataContext="{Binding Steps}" ItemsSource="{Binding Path=/Elements, Mode=TwoWay  }"        Grid.RowSpan="3">
        <ListView.View>
            <GridView>
                <GridViewColumn Header="Name" DisplayMemberBinding="{Binding Path=Attribute[name].Value}"   />
            </GridView>
        </ListView.View>
    </ListView>
    <GridSplitter HorizontalAlignment="Left" Height="100" VerticalAlignment="Top" Width="6" Grid.Column="1" Margin="2.6,80,0,0" Grid.Row="2"/>
    <WrapPanel Grid.Row="1"  Grid.Column="1">
        <Label Content="StepID:" Grid.Column="1" HorizontalAlignment="Left" Grid.Row="1" VerticalAlignment="Top" />
        <Label Content="{Binding Path=Attribute[stepID].Value}" HorizontalAlignment="Left" VerticalAlignment="Top" DataContext="{Binding SelectedItem, ElementName=listView1, Mode=TwoWay }" />
    </WrapPanel>
    <TextBox  Grid.Column="1" Grid.Row="2" Grid.ColumnSpan="2" Grid.RowSpan="3"  Margin="1,0,0,0"
               Text="{Binding SelectedItem, ElementName=listView1, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged }"  TextWrapping="Wrap" AcceptsReturn="True"  >
    </TextBox>

Elements is a readonly property, however I see no way around this as when attempting to make calls without linq to childnodes or descendants it returns null.