Have you considered using a Repeater instead of a GridView? The Repeater is probably the easiest way to have 2 rows per record, you get more control over the tabular markup that gets generated e.g.
<asp:Repeater runat="server" ID="Repeater1" >
<HeaderTemplate>
<table>
<tr>
<th>Field 1</th>
</tr>
<tr>
<th>Field 2</th>
</tr>
</HeaderTemplate>
<ItemTemplate>
<tr>
<td><%# DataBinder.Eval(Container.DataItem, "field1") %></td>
</tr>
<tr>
<td><%# DataBinder.Eval(Container.DataItem, "field2") %></td>
</tr>
</ItemTemplate>
<FooterTemplate>
</table>
</FooterTemplate>
</asp:Repeater>