I work in a larger facility that determines what I charge; however we charge 25.00 per person. A typical session is 45.00-55.00 so each person is paying half roughly. My gym does require that both be there to train or the one inidividual will pay the full price (unless canceled).
My personal trainer does training from his home and he does pretty close to the same. He takes his session price and charges that regardless if it is one or two. So if my boyfriend and I train together we split the cost.
At my old gym where I determined the price – an hour session for one was 40.00, I went up to 50.00 for the two. Simply because a little more prep goes into the workout.
Just a few ways I have seen it done in my area.