Having done this on NW, as well as KL and AF, I'd advise against trying anything tricky. If you want to book 2 seats for 1 passenger, do it over the phone. If you get an agent who says (s)he has never done it before, ask to be transferred to one who has. It can go wrong quite easily, with the reservation agent, the gate agent or the purser.

If you do as you propose and book 2 separate tickets, I predict that you won't get the 2 seats on your outbound and that one of your returns will be voided.