There is no 'COLSPAN' for modules. Just used it as a comparison.
Anyway, what you should do, kind of, is make a skin with following 'main' section
<table>
<tr>
<td ID="SpanPane" runat="server" visible="false" colspan=2></td>
</tr>
<tr>
<td ID="LeftPane" runat="server" visible="false"></td>
<td ID="RightPane" runat="server" visible="false"></td>
</tr>
</table>
If you upload this kind of skin, you will get a pane in the top row (called SpanPane) and two panes next to each other in the bottom row (called LeftPane and RightPane).
You should add your 'spanning' module to the SpanPane, and the other modules to the other pane(s).
You can extend this solution with other panes up to a maximum of 10 panes.