PREFIX : <http://example.org/ns#>

# Replace OPTIONAL with "UNION TRUE"

SELECT *
{
  # Use UNION to generate both possibilities in the same query.
  { { :a :b ?x } UNION {} . { :d :e ?x } UNION {} }
##   UNION
##   { { :d :e ?y } UNION {} . {  :a :b ?y } UNION {} }
}