XSLT: Testing position against concurrent criteria within xsl:for-each - xslt

Question

Consider the following two xsl:value-of statements using XPath expressions testing the position of a node.
(1) This one, as I understand it, will return the value of the /ChildThree child of a /Parent where three things are independently true of that /Parent - (i) it has a /ChildOne with value "x"; AND (ii) it has a /ChildTwo with value "y"; AND (iii) it is the second or subsequent /Parent within its immediate ancestor:
<xsl:value-of select="Parent[ChildOne='x'][ChildTwo='y'][position()>=2]/ChildThree"/>
(2) This one, by contrast, will return the value of the /ChildThree child of a /Parent where that /Parent is the second or subsequent /Parent OF THOSE THAT HAVE BOTH a /ChildOne with value "x" AND a /ChildTwo with value "y":
<xsl:value-of select="(Parent[ChildOne='x'][ChildTwo='y'])[position()>=2]/ChildThree"/>
So far so good. However, what happens in the following example? Here, I'm trying to get the value of the /ChildThree child of any /Parent for which the first two criteria are concurrently true, but only precede it with a space if the /Parent is the second or subsequent within that subset (i.e., like example (2), above). How can I dictate how the position criterion within the xsl:if statement applies to the criteria in the xsl:for-if statement, when it is not part of the same XPath expression?
<xsl:for-each select="Parent[ChildOne='x'][ChildTwo='y']">
<xsl:if test="position()>=2">
<xsl:text> </xsl:text>
</xsl:if>
<xsl:value-of select="ChildThree"/>
</xsl:for-each>

Answer 1

The two statements (1) and (2) are actually equivalent, and will return the same results.
position() is context-sensitive. It will give you the position of the current node within the set of nodes previously selected. So, when you have the expression Parent[ChildOne='x'][ChildTwo='y'] it is returning a set of nodes where two conditions is true. Doing Parent[ChildOne='x'][ChildTwo='y'][position()>=2] will give the second (or greater) node in this list.
The way you are describing how you think (1) works, would actually look like this.
<xsl:value-of select="Parent[position()>=2][ChildOne='x'][ChildTwo='y']/ChildThree"/>
One way to think of it, is to think of every condition in square brackets filtering what has come before.
Try it out on this XML, for example
<Parents>
<Parent>
<ChildOne>a</ChildOne>
<ChildTwo>b</ChildTwo>
<ChildThree>c</ChildThree>
</Parent>
<Parent>
<ChildOne>x</ChildOne>
<ChildTwo>y</ChildTwo>
<ChildThree>z1</ChildThree>
</Parent>
<Parent>
<ChildOne>x</ChildOne>
<ChildTwo>y</ChildTwo>
<ChildThree>z2</ChildThree>
</Parent>
</Parents>
Your statements (1) and (2) both return z2, but doing Parent[position()>=2][ChildOne='x'][ChildTwo='y']/ChildThree returns z1.
What this means is that your xsl:for-each should actually give you the results you expect.