It is for either one or two people, no difference. The only difference if you add that second person is an added low-bucket coach fare.
I think you are asking because you want to add a friend who will board later? There are others who know more than I do about this -- but it's my take on what I've read on this board that friend then buys an "open sleeper ticket", and friend needs to provide your name and reservation number (?) to tie their "open sleeper ticket" to your sleeper reservation. There will be no addition to the sleeper upcharge on your ticket, and no sleeper charges for your friend. If I have it wrong, someone please correct me! Of course, I want to know how to do this because I'm thinking of exactly the same scenario on MY next trip, LOL.