make all tables span a multi-column layout

If I want to make all tables in my document span a 2-column layout, what 
do I need to set? I've tried:

     <xsl:attribute-set name="table.table.properties">
         <xsl:attribute name="span">all</xsl:attribute>
         <xsl:attribute 
name="keep-together.within-column">always</xsl:attribute>
     </xsl:attribute-set>


But no effect. Tables remain in the single column?

Thanks,

--Scott