DOCBOOK-APPS: generating a two column index

I am trying to generate a two column index in a book, whose pages 
are formatted as single columns. My customization layer has the 
following setting:

<xsl:param name="column.count" select="'1'"/>

I tried modifying the following template in index.xsl without any 
success. Here's one attempt that I made:

<xsl:template match="book/index">
  <xsl:variable name="master-name">
    <xsl:call-template name="select.pagemaster">
	<xsl:with-param name="column.count" select="'2'"/>
	</xsl:call-template>
  </xsl:variable>

I would appreciate any advice or suggestions on how I can get a 
two column index to work. 

Thanks in advance, 

Jeff