Working with multi-schema XML files

The XML files we have worked with so far were straightforward and only dealt with a single collection of elements. However, many systems produce or consume XML files that contain multiple collections of elements; these are called multi-schema XML files. Here's an example:

<?xml version="1.0" encoding="UTF-8"?>

<catalogue>
  <skus>
    <sku>
      <skuid>432345</skuid>
      <skuname>Check Shirt</skuname>
      <size>S</size>
      <colour>Green</colour>
      <price>29.99</price>
    </sku>
  </skus>
  <inventory>
    <sku>
      <skuid>432345</skuid>
      <stock_on_hand>12</stock_on_hand>
    </sku>
  </inventory>
</catalogue>

This shows a product catalogue file with two schemas, one for the product details and one for the inventory. There's nothing in the XML structure to connect the two schemas. In essence...