then i need to have headers like these:
page 1: title 1 - title 2
page 2: title 2 - title 3
page 3: title 3
page 4: title 3 - title 4

but so far what i get is this:

page 1: title 1 - title 2
page 2: title 2 - title 3
page 3: title 3 - title 3
page 4: title 3 - title 4

You can try to define an empty heading-end marker after the
heading start, and the real heading-end right before the
next heading start. Be careful to have a proper heading end
at the end of the document.

 <xsl:template match="article">
    <fo:block>
       <fo:marker marker-class-name="heading-start"><xsl:value-of
 select="head"/></fo:marker>
       <fo:marker marker-class-name="heading-end" />
       <xsl:apply-templates/>
       <fo:marker marker-class-name="heading-end"><xsl:value-of
 select="concat(' - ', head)"/></fo:marker>
    </fo:block>
 </xsl:template>