Here's a very simple way to look at it:
XSLT 1.0
<?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:template match="/">
<ul>
<xsl:for-each select="main/part/block">
<xsl:variable name="i" select="position()" />
<li><xsl:value-of select="."/></li>
<li><xsl:value-of select="../../article/block[$i]"/></li>
</xsl:for-each>
</ul>
</xsl:template>
</xsl:stylesheet>