 It is standard practice to allow for the possibility of total failure of gas supply from any one cylinder at any point of the dive, and to turn the dive if the gas supply is compromised beyond the planned limits.Most technical and overhead-environment divers will maintain the rule of thirds as the least conservative option when diving sidemount.Because air-sharing will remain a possibility while a second diver is present, it is considered good practice that both cylinders retain independent reserves of sufficient gas for both divers to reach the surface safely from any point in the planned dive.