master.xml

<?xml version="1.0" encoding="utf-8" ?>
<data>
  <params>
    <root>http://www.example.com</root>
    <view />
  </params>
  <projectpreviews>
    <section id="1" handle="projects">Projects</section>
    <entry id="2">
      <project-name handle="sample-project-2">Sample Project 2</project-name>
      <project-description mode="formatted"><p>desc 2</p></project-description>
      <project-preview mode="formatted"><p>d2</p></project-preview>
      <project-button-text handle="example-link">Example Link</project-button-text>
      <project-button-URL handle="http-example">http://example.com/</project-button-URL>
      <project-button-icon handle="icon-example">icon-example</project-button-icon>
    </entry>
    <entry id="1">
      <project-name handle="sample-project">Sample Project</project-name>
      <project-description mode="formatted"><p>desc 1</p></project-description>
      <project-preview mode="formatted"><p>d1</p></project-preview>
    </entry>
    <entry id="3">
      <project-name handle="sample-project">Sample Project 3333</project-name>
      <project-description mode="formatted"><p>desc 1</p></project-description>
      <project-preview mode="formatted"><p>d1</p></project-preview>
    </entry>
  </projectpreviews>
</data>

master.xsl

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">

<xsl:output method="xml" indent="yes" />

<xsl:template match="/">
  <xsl:apply-templates select="data/projectpreviews/entry[position() mod 2 != 0 and position() != last()]" mode="two-cols" />
  <xsl:apply-templates select="data/projectpreviews/entry[position() = last() and not(position() mod 2 = 0)]" mode="single-col" />
</xsl:template>

<xsl:template match="entry" mode="two-cols">
  <div class="row">
    <xsl:apply-templates select=". | following-sibling::entry[1]" mode="column" />
  </div>
</xsl:template>

<xsl:template match="entry" mode="single-col">
  <div class="full-span-row">
    <xsl:apply-templates select="." mode="project" />
  </div>
</xsl:template>

<xsl:template match="entry" mode="column">
  <div class="col">
    <xsl:apply-templates select="." mode="project" />
  </div>
</xsl:template>

<xsl:template match="entry" mode="project">
  <h3><xsl:value-of select="project-name" /></h3>

  <xsl:if test="project-button-text">
    <p>Buttons for free</p>
    <a class="btn pull-right" href="{project-button-URL}">
      <xsl:value-of select="project-button-text" />
    </a>
  </xsl:if>
</xsl:template>

</xsl:stylesheet>