So, if the altered range elements are in $split,

<xsl:variable name="pairs">
    <xsl:perform-sort select="$split/range/start | $split/range/end" >
        <xsl:sort select="."/>
    </xsl:perform-sort>
</xsl:variable>

and then you can consume $pairs pair-wise from the front, using
subsequence(), to get that actual ranges.

(XSLT has not been tested.)

Did that make any sense at all?

-- Graydon