Had that issue on BA some years ago: They would not allow two reservations on the same flight in the first place. They didn't mind, however, putting in one reservation as D-AIPD/John and the other as D-AIPD/J
But as for LH: Didn't this forum discuss, a while ago, how to book two seats (and obtain double mileage)? How would that work if they don't like double reservations?