Add products of two recursive additions

Hello
I have two recursive addition functions producing totals which I would like
to add together.  How can I do this?

Eg To produce the first total I have used the xsl shown below:

<xsl:template match="a">
<xsl:call-template name="orderlist.sum.edit">
<xsl:with-param name="set-of-order" select="b/c"/>
</xsl:call-template>
</xsl:template>

The call-template above calls the template below to display the total for
'data':

<xsl:template name="orderlist.sum.edit">
<xsl:param name="set-of-order"/>
<xsl:variable name="sumorg">
<xsl:call-template name="orderlist.sum">
<xsl:with-param name="set-of-order" select="$set-of-order"/>
</xsl:call-template>
</xsl:variable>
<xsl:value-of select='format-number($sumorg, "£###,###,##0.00")'/>
</xsl:template>

<xsl:template name="orderlist.sum">
<xsl:param name="set-of-order"/>
<xsl:choose>
<xsl:when test="$set-of-order">
<xsl:variable name="first">
<xsl:apply-templates select="$set-of-order[1]/data"/>
</xsl:variable>
<xsl:variable name="rest">
<xsl:call-template name="orderlist.sum">
<xsl:with-param name="set-of-order" select="$set-of-order[position() !=
1]"/>
</xsl:call-template>
</xsl:variable>
<xsl:value-of select="$first + $rest"/>
</xsl:when>
<xsl:otherwise>0</xsl:otherwise>
</xsl:choose>
</xsl:template>

The second total is acquired in the same way.  An example of the xml I am
using is also shown below:

<a>
 <b>
  <c>
    <data>1</data>
  </c>
   <c>
    <data>3</data>
  </c>
  <d>
    <data>1</data>
  </d>
  <d>
   <data>5</data>
  </d>
  <d>
   <data>2</data>
  </d>
 </b>
</a>

The two call-templates above refering to the recursive addition functions
produce the totals for <data> in <c> as 5 and <d> as 7. How do I obtain 12
as the total of both?

Thankyou very much for your help.