How do I bind element widths where "Auto" width behaviour is required?

Question

I wish to have a set of elements have the same width, I can not simply put them in a grid as this is a content control and the elements being bound is unknown.

As a simple example, how do I bind all widths of the first child of each stack panel together along with the second element.

  <StackPanel>
    <StackPanel Orientation="Horizontal">
      <Label Content="First column"/>
      <Label Content="Second Column" Background="Green"/>
    </StackPanel>
    <StackPanel Orientation="Horizontal">
      <Label Content="One"/>
      <Label Content="Two" Background="Green"/>
    </StackPanel>
  </StackPanel>

Answer 1

    <StackPanel>
        <StackPanel Orientation="Horizontal">
            <Label x:Name="FirstColumnLabel" Content="First column"/>
            <Label x:Name="SecondColumnLabel" Content="Second Column" Background="Green"/>
        </StackPanel>
        <StackPanel Orientation="Horizontal">
            <Label Content="One" Width="{Binding ElementName=FirstColumnLabel, Path=ActualWidth}"/>
            <Label Content="Two" Width="{Binding ElementName=SecondColumnLabel, Path=ActualWidth}" Background="Green"/>
        </StackPanel>
    </StackPanel>

Answer 2

I'm not clear on your reason for not using a Grid from the sample code here but Grid gives you a built-in mechanism to do this:

 <StackPanel Grid.IsSharedSizeScope="True">
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="Auto" SharedSizeGroup="Col1"/>
            <ColumnDefinition Width="Auto" SharedSizeGroup="Col2"/>
        </Grid.ColumnDefinitions>
        <Label Content="First column" Grid.Column="0"/>
        <Label Content="Second Column" Background="Green" Grid.Column="1"/>
    </Grid>
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition SharedSizeGroup="Col1"/>
            <ColumnDefinition SharedSizeGroup="Col2"/>
        </Grid.ColumnDefinitions>
        <Label Content="One" Grid.Column="0"/>
        <Label Content="Two" Background="Green" Grid.Column="1"/>
    </Grid>
</StackPanel>

Will this work for you or do you have other constraints that won't allow you to do this?