require 'rexml/document'

doc = REXML::Document.new <<EOF
  <company>
    <employee name='tom'   secretary='john'/>
    <employee name='dick'  assistant='jane'/>
    <employee name='harry' assistant='jim' secretary='bob'/>
  </company>
EOF

p doc.root.elements.to_a('employee[@secretary]')
p doc.root.elements.to_a('employee[@assistant]')
p doc.root.elements.to_a('employee[@secretary and @assistant]')