 It is not safe practice to drain one cylinder, retaining the entire reserve in the other, in case of a catastrophic failure of the second cylinder supply.Sidemount divers are trained to maintain a roughly equal balance of gas supply in each primary cylinder throughout the dive.This ensures that they always retain an equal reserve of gas for donation to an out-of-gas diver; and also helps preserves their stability and lateral trim by maintaining equal buoyancy characteristics in the tanks on either side of their body.