<ROOT>
<A>
  <D N="" Y="" />
  <D N="" Y="" />
  <D N="" Y="" />
  <D N="" Y="" />
...............
</A>

<B>
  <D N="" Y="" />
  <D N="" Y="" />
  <D N="" Y="" />
  <D N="" Y="" />
...............
</B>
</ROOT>

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
	 <xsl:output method="xml" version="1.0" encoding="UTF-8" indent="yes"/>

     <xsl:variable name="set1" select="//A/D" />
     <xsl:variable name="set2" select="//B/D" />	

     <xsl:template  match="ROOT"> 
		 <ROOT>
			<xsl:call-template  name="AGE" />
		 </ROOT>
	 </xsl:template>  

     <xsl:template name="AGE" match="ROOT"> 
		 <ROOT>
			 <xsl:copy-of select="$set2[not(@N=$set1/@N)]  | $set1[@Y > 2] "  /> 		 
         </ROOT>
	 </xsl:template>  

</xsl:stylesheet>

<xsl:variable name="set1" select="ROOT/A/D[@Y > 2]" /> 
   
  <xsl:key name="skey1" match="ROOT/A/D" use="@N" />
  <xsl:key name="skey2" match="ROOT/B/D" use="@N" />
           
   <xsl:template  match="ROOT"> 
	 <ROOT>
		<xsl:call-template  name="AGE" />
	 </ROOT>
    </xsl:template>  

     <xsl:template name="AGE" match="ROOT"> 
	 <ROOT>
	 	 <xsl:for-each select="//ROOT/B/D">
			 <xsl:if test="count(key('skey1', @N))=0" >
				<xsl:copy-of select="key('skey2', @N)"  />
			 </xsl:if>	 
		 </xsl:for-each>
		 <xsl:copy-of select="$set1"  />
         </ROOT>
	 </xsl:template>