make a (real) minimal example and we'll see what can be done
Here it is:
\startbuffer[test] <body> <p>In this paragraph<footnote>first footnote</footnote><endnote>endnote strictly following a footnote (no other XML nodes between them)</endnote> a footnote marker and an endnote marker are at the same point in the text; in the XML source, their elements are contiguous, with nothing in between. The CSS selector <code>footnote + endnote</code> works right, and the setup associated to that selector puts a comma between their markers to separate them in the typesetting.</p>
<p>In this paragraph<footnote>second footnote</footnote> they don't occur at the same point in the text<endnote>endnote comes after footnote: there's some text between them, but no XML elements</endnote>; for the CSS selector <code>footnote + endnote</code> it's the same case of the previous paragraph, because there's no other XML element between the <code>footnote</code> and the <code>endnote</code> elements: that's why you still see a comma before the endnote's marker, but this time it's not what I want.</p>
<p>In this paragraph<footnote>third footnote</footnote> they don't occur at the same point in the text, and the CSS selector <code>footnote + endnote</code> does not match<endnote>endnote comes after footnote, but there are 3 nodes between them: a text, a <code>code</code> element, another text</endnote>, because there's a <code>code</code> element between <code>footnote</code> and <code>endnote</code> elements.</p>
<p>CSS has no operator to discriminate between the first two cases.</p> </body> \stopbuffer
\setupnotation[footnote][numberconversion=a] \setupwhitespace[line]
\startxmlsetups xml:notesetups \xmlsetsetup{#1}{body|p|footnote|code}{xml:*} \xmlsetsetup{#1}{{endnote}}{xml:endnote} \xmlsetsetup{#1}{{footnote + endnote}}{xml:endnote-after-footnote} \stopxmlsetups
\xmlregistersetup{xml:notesetups}
\startxmlsetups xml:body \xmlflush{#1} \stopxmlsetups
\startxmlsetups xml:p \xmlflush{#1}\par \stopxmlsetups
\startxmlsetups xml:footnote \footnote{\xmlflush{#1}} \stopxmlsetups
\startxmlsetups xml:code {\tt\xmlflush{#1}} \stopxmlsetups
\startxmlsetups xml:endnote \endnote{\xmlflush{#1}} \stopxmlsetups
\startxmlsetups xml:endnote-after-footnote \high{,}\endnote{\xmlflush{#1}} \stopxmlsetups
\starttext \xmlprocessbuffer{main}{test}{} \page \placenotes[endnote] \stoptext