Nested grid column width inheritence

I have:

<Grid><Grid.ColumnDefinitions><ColumnDefinition Width="Auto"/><ColumnDefinition Width="*"/></Grid.ColumnDefinitions><!-- stuff in both columns --><Grid Grid.Row="2" Grid.Column="0" ColumnSpan="2"><Grid.ColumnDefinitions><ColumnDefinition Width="Auto"/><ColumnDefinition Width="*"/></Grid.ColumnDefinitions><!-- stuff in second grid --></Grid></Grid>

I want the left column of the second grid to have the same width as the left column in the first grid. How do I do this?

What is currently happening is the nested grid is column width is smaller than the parent one.

Any questions just ask,

Thanks,

d3