There really needs to be two, preferably three roll bars in a tandem aircraft. One in front of the pilot, one between the pilot and passenger and one behind the passenger. The middle one is not as theoretically essential as the other two but you want as much width of roll bar as possible to allow the weight of the aircraft to be spread and to avoid it simply bending (or snapping off its attachments) during an overturn. Especially on soft soil, a thin roll bar offers very little protection because the weight of the aircraft and the momentum will simply cause it to dig in and you end up having much the same effect as no roll bar at all. I have in my research files, a case from the late 1980s where the pilot was found in his aircraft with a lot of injuries- none of which would have been rapidly fatal- but his mouth and nose were packed full of soft mud from the field he had overturned in. The death was still listed as due to multiple injuries but a contributing factor was asphyxiation. Also the roll bars should ideally be continuous down to the floor of the aircraft so as to minimize the chance of them being leveraged off during a crash and also help to reinforce the cockpit to give some added protection to the rest of the occupants' bodies.