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>