How can I merge the following two XML documents:

Persons.xml
<?xml version="1.0" encoding="UTF-8"?>
<persons>
  <person>
    <name>Albert</name>
    <department_id>1</department_id>
  </person>
  <person>
    <name>Barry</name>
    <department_id>2</department_id>
  </person>
  <person>
    <name>Charlie</name>
    <department_id>3</department_id>
  </person>
</persons>

Departments.xml
<?xml version="1.0" encoding="UTF-8"?>
<departments>
  <department>
    <id>1</id>
    <name>Management</name>
  </department>
  <department>
    <id>2</id>
    <name>Sales</name>
  </department>
  <department>
    <id>3</id>
    <name>Support</name>
  </department>
</departments>

To generate the following XML:

<?xml version="1.0" encoding="UTF-8"?>
<persons>
  <person>
    <name>Albert</name>
    <department>Management</department>
  </person>
  <person>
    <name>Barry</name>
    <department>Sales</department>
  </person>
  <person>
    <name>Charlie</name>
    <department>Support</department>
  </person>
</persons>