Common stylesheet
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" />
<xsl:import href="stylesheet1.xsl" />
<xsl:import href="stylesheet2.xsl" />
<xsl:param name="processing-mode" />
<xsl:template match="/">
<xsl:choose>
<xsl:when test="$processing-mode = 'mode1">
<xsl:apply-templates select="." mode="stylesheet1-mode" />
</xsl:when>
...
<xsl:otherwise>
<xsl:apply-templates select="." mode="default-mode" />
</xsl:otherwise>
</xsl:choose>