Still feeling brain dead on XAML :(

Simple control:

<UserControl x:Class="cdsThemes.TestControl2"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Height="300" Width="300">
    <Grid Background="BlanchedAlmond">
  <TextBox Height="23" 
             Margin="25,103,0,0" 
             Name="textBox1" 
             VerticalAlignment="Top" 
             HorizontalAlignment="Left" 
             Width="120">Blah Blah Blah</TextBox>
  </Grid>
</UserControl>

And I use it in another simple control:

<UserControl x:Class="cdsThemes.TestControl1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:cdsThemes"
    Height="300" Width="300">
    <Grid>
    <local:TestControl2 x:Name="testControl2"></local:TestControl2>
  </Grid>
</UserControl>

Lets say I want to change the margin on textBox1 inside of TestControl2 from the XAML in TestControl1. How do I do that?

My actual situation may be harder because textBox1 in that case is a custom control based on a TextBox defined in another assembly. But I can't even get this one to work, so I figured I'd start with the simpler problem.

I can do it easy in C#

testControl2.textBox1.Margin = new Thickness(1, 1, 1, 1);

just can't seem to work out the XAML.