I don't own a J315, but I did buy a Hot Spring Jetsetter in January which is almost exactly the same size and general layout (the J315 is a couple of inches deeper and a couple of inches shorter). With my tub it is usally just me in the tub, but on the occasions that there have been 2 people in it, I have felt there is enough room for comfort, however ability to rotate from seat to seat is limited, and requires a certain amount of coordination. One issue I have experienced that I did not expect, but should have, is the problem with changing water level when there is one vs two people in such a small tub. Simply put when the water is so low as to almost expose the neck jets when no one is in the tub, it will be about right with 1 person, and near overflowing if you add a second person, and I can't imagine what would happen with a third. So as to size, it works for me as a 1 person, or occasional 2 person tub, but it was always going to be more than just me I would probably want something a little bigger, so I could change seating (types of jets) without forcing the other person to move..