Conditional Grouping Problem

Hi all,

I have an interesting grouping issue.  I have xml like:

<product>
        <buyer>John Q. Public</buyer>
        <name>Our Best Widget</name>
</product>
<product>
        <buyer>John Dow</buyer>
        <name>Our Best Widget</name>
</product>
<product>
        <buyer>John Q. Public</buyer>
        <name>Our Worst Widget</name>
</product>
<product>
        <buyer>Jane Doe</buyer>
        <name>Our Best Widget</name>
</product>
<product>
        <buyer>Jill Doe</buyer>
        <name>Our Worst Widget</name>
</product>


I am trying to to group in 2 sections.
section 1: by product name
section 2: by product buyer

Here's the rub, section 1 is for all buyers who only purchased 1 
product.  Section 2 is for all buyers who buy multiple products.  The 
final output should look like:
<!-- single buyers by product name -->
<name>Our Best Widget</name>
        <buyer>Jane Doe</buyer>
        <buyer>John Doe</buyer>
<name> Our Worst Widget</name>
        <buyer>Jill Doe</buyer>

<!-- multiple buyers by buyer -->
<buyer>John Q. Public</buyer>
        <name>Our Best Widget</name>
        <name>Our Worst Widget</name>


If we focus on section 1 for now, I'm using the Muenchian Method with a 
key like:
<xsl:key name="product-by-name" match="Product" use="name" />
How can I modify this key to match only products which are part of a 
single order (the Product's buyer only bought 1 item)?
I'm stretching my xsl knowledge here..

I assume if i can figure this out i can use the same method for section 
2.
Thanks for any help,
-jonathan