Construct HXT arrow that selects multiple Maybe values

Question

I'm confused with HXT arrows API. Similarly to the list monad, any arrow that produces no result will determine result of [] and match will become element in one dimension of Cartesian product of all matches.

I have XML file with a nested optional elements like this:

<root>
 ...
 <b>
  <x>1</x>
  <y>2</y>
  ...
 </b>
 <x>3</x>
 <y>4</y>
 ...
</root>

How can I construct an arrow that will traverse this tree once and produce result in fixed structure similar to one below? I care only about small part of the structure.

[(Just 1, Just 2), (Just 3, Just 4)] :: [(Maybe a, Maybe b), (Maybe a, Maybe b)]

or

[(Nothing, Nothing), (Just 3, Just 4)] :: [(Maybe a, Maybe b), (Maybe a, Maybe b)]