BTemplate 2 Iterations

Hi,

I have to populate a template with data from two iterations
into a table.

Group-Description 0
   Detail-Row 0,0
   Detail-Row 0,1
   Detail-Row 0,2
Group-Description 1
   Detail-Row 1,0
   Detail-Row 1,1
Group-Description 2
   Detail-Row 2,0
   Detail-Row 2,1
   Detail-Row 2,2
   Detail-Row 2,3

Data for Group Row and Detail Row comes from the same ViewModel,
stepping throug the Iteration. The Template looks like this:

<!-- Group Row -->
<!-- Problem, how two iterate this one ? -->
  <tr>
    <th colspan="2" align="left">GROUP_DATA</th>
    <th align="left">4712</th>
    <td>&nbsp;</td>
   </tr>

<!-- DetailRow -->
<tr class="Dir::Iterate_Start.DownloadViewModel 
Dir::Iterate_Next.DownloadViewModel">
       <td>&nbsp;</td>
       <td><a href=""><span 
class="Dir::Get_Data.DownloadViewModel.filename">name</span></a></td>
       <td><span 
class="Dir::Get_Data.DownloadViewModel.creationdate">datum</span></td>
       <td><span 
class="Dir::Get_Data.DownloadViewModel.size">groesse</span></td>
   </tr>
   <tr class="Dir::Iterate_End.DownloadViewModel 
Dir::Discard"><td></td></tr>

What is the best way to get the data of the Group Row into the
template?

For nice ideas, thanks in advance,

Carsten