You can't do this with the standard JSF implementation. In JSF 1.2 one would have used Tomahawk's <t:panelGroup colspan="2"> for this. Right now Tomahawk is not officially compatible with JSF 2.0, but I just gave it a try.
<html xmlns:t="http://myfaces.apache.org/tomahawk"> ... <t:panelGrid columns="3"> <t:panelGroup>row1cell1</t:panelGroup> <t:panelGroup>row1cell2</t:panelGroup> <t:panelGroup>row1cell3</t:panelGroup> <t:panelGroup colspan="2">row2cell1-2</t:panelGroup> <t:panelGroup>row2cell3</t:panelGroup> <t:panelGroup>row3cell1</t:panelGroup> <t:panelGroup colspan="2">row3cell2-3</t:panelGroup> <t:panelGroup>row4cell1</t:panelGroup> <t:panelGroup>row4cell2</t:panelGroup> <t:panelGroup>row4cell3</t:panelGroup> </t:panelGrid>
And it works. I don't guarantee that other Tomahawk components will work as well.