So, if I follow your logic, no telephone conversation between people in (say) Paris and New York can possibly be considered transatlantic since the communication goes both ways and by your definition trans can only be one-way traffic. Alternatively, if we accept that the conversation is indeed transatlantic, then a flight carrying an individual from Washington to London would not be transatlantic because it only crosses the Atlantic in one direction, not both.
What description do you propose we use to differentiate these two concepts (in my examples) so that Trans does not merely mean "cross" but means either "crossing and returning" or "crossing in a single direction"?