 Sidemount diving with two equal-sized cylinders helps resolve stability and streamlining issues, and can ensure that an adequate redundant reserve of air is maintained, similar to back-mounted twins.Back-mounted manifolded cylinders provide easy access to the whole gas supply in the event of a regulator failure and shut-down.However, the manifold itself creates additional potential o-ring failure points, and a major leak at the manifold will deprive the diver of at least one-half of the remaining gas supply.