If your structure is built in the same way as above, this XPath works fine:
var _root = app.activeDocument.xmlElements.firstItem();
var _firstXrefID001andID002 = _root.evaluateXPathExpression("//p[position()=1]/xref[@rid='id001']|/ /p[position()=2]/xref[@rid='id002']");
app.select(_firstXrefID001andID002);
If other positions of <p> are possible, e.g.
<DOCUMENT>
<CN>1</CN>
<CT>Title</CT>
<p1>Sample Text Sample Text Sample Text Sample Text Sample Text Sample Text</p1>
<p>Sample Text Sample Text Sample Text <xref rid="id001" ref-type="Test"/> Sample Text Sample Text Sample Text</p>
<p>Sample Text Sample Text Sample Text <xref rid="id001" ref-type="Test"/> Sample Text Sample Text Sample Text</p>
<p>Sample Text Sample Text Sample Text <xref rid="id001" ref-type="Test"/> Sample Text Sample Text Sample Text</p>
<p>Sample Text Sample Text <xref rid="id002" ref-type="Test"/> Sample Text Sample Text Sample Text Sample Text</p>
<p>Sample Text Sample Text <xref rid="id002" ref-type="Test"/> Sample Text Sample Text Sample Text Sample Text</p>
<p>Sample Text Sample Text <xref rid="id002" ref-type="Test"/> Sample Text Sample Text Sample Text Sample Text</p>
<p>Sample Text Sample Text <xref rid="id001" ref-type="Test"/> Sample Text Sample Text Sample Text Sample Text</p>
<p>Sample Text Sample Text <xref rid="id001" ref-type="Test"/> Sample Text Sample Text Sample Text Sample Text</p>
<p>Sample Text Sample Text <xref rid="id001" ref-type="Test"/> Sample Text Sample Text Sample Text Sample Text</p>
<p>Sample Text Sample Text <xref rid="id001" ref-type="Test"/> Sample Text Sample Text Sample Text Sample Text</p>
<p>Sample Text Sample Text <xref rid="id002" ref-type="Test"/> Sample Text Sample Text Sample Text Sample Text</p>
<p>Sample Text Sample Text <xref rid="id002" ref-type="Test"/> Sample Text Sample Text Sample Text Sample Text</p>
<p>Sample Text Sample Text <xref rid="id002" ref-type="Test"/> Sample Text Sample Text Sample Text Sample Text</p>
<p>Sample Text Sample Text <xref rid="id002" ref-type="Test"/> Sample Text Sample Text Sample Text Sample Text</p>
<p>Sample Text Sample Text <xref rid="id002" ref-type="Test"/> Sample Text Sample Text Sample Text Sample Text</p>
</DOCUMENT>
You can try these XPathsExpressions and take the first element of resulting Arrays:
var _root = app.activeDocument.xmlElements.firstItem();
var _firstXrefID001 = _root.evaluateXPathExpression("//p/xref[@rid='id001']");
app.select(_firstXrefID001[0]);
var _firstXrefID002 = _root.evaluateXPathExpression("//p/xref[@rid='id002']");
app.select(_firstXrefID002[0]);
Roland