You can name the RotateTransform and then bind to its properties.  For example, in your 'main' ui element, you define the transform as so:
<TextBlock Text="MainBox">
  <TextBlock.RenderTransform>
    <RotateTransform Angle="20" 
                     CenterX="50" 
                     CenterY="50" 
                     x:Name="m"/>
  </TextBlock.RenderTransform>
</TextBlock>
Then you can bind to that transform from another element:
<TextBlock Text="SecondBox">
  <TextBlock.RenderTransform>
    <RotateTransform Angle="{Binding Angle, ElementName=m}"
                     CenterX="{Binding CenterX, ElementName=m}" 
                     CenterY="{Binding CenterY, ElementName=m}"/>
  </TextBlock.RenderTransform>
</TextBlock>