That's all you have to do. I do this all the time. You WILL NOT get OP miles and you can get extra baggage.
Sometimes an inexperienced GA or FA will give you an odd look but the simple truth will placate them. Watch out for equipment changes or schedual change--they might separate you. Be prepared to call immediately and howl if this happens. Invariably, they correct it. Chances of this happening are less if
you make both reservations at the same time.
You have to watch out for poachers of course. In my experience, a firm "that seat is paid for" is all that's necessary.
Considering the wretched pitch in CO economy seating, this is really a viable option to make a long flight tolerable.

Matt