xml = XMLObject["Document"][
  XMLElement[
   "whatever", {}, {
     {XMLElement["forename", {"type" -> "first"}, {"Adrian"}],
      XMLElement["surname", {}, {"Adgar"}]
    },
    {XMLElement["forename", {"type" -> "first"}, {"Babar"}],
     XMLElement["forename", {"type" -> "middle"}, {"Barbados"}],
     XMLElement["surname", {}, {"Bode"}]
    },
    {XMLElement["forename", {"type" -> "first"}, {"Charly"}],
     XMLElement["surname", {}, {"Chimp"}]
    }
}]]

Cases[xml, {(XMLElement["forename", _, x_])...,
      XMLElement["surname", {}, y_]} -> {x, y}, Infinity]
(* {{{"Adrian"}, {"Adgar"}}, {{"Charly"}, {"Chimp"}}} *)