> What I can't seem to accomplish is sorting the resulting 
> pairs into numerical order based on the first and then the 
> second values in each pair. 

looks to me like

<xsl:for-each...
  <xsl:sort select="number(substring-before(., ' '))" data-type="number"/>
  <xsl:sort select="number(substring-after(., ' '))" data-type="number"/>

Michael Kay
http://www.saxonica.com/