In this configuration, two cylinders of equal capacity are banded together with stainless steel or webbing bands and attached to a stainless steel or aluminium back plate with a buoyancy compensating wing sandwiched in between.

Each cylinder is fitted with its own regulator, each with a second stage and a submersible pressure gauge (SPG). One regulator will also carry the low pressure inflator for the wing BC, and, if a dry suit is worn, the other will carry the low pressure inflator for the dry suit.

The diver progressively breathes down each cylinder by swapping second stages, keeping the cylinders roughly equal until the turnaround pressure is reached.

Advantages

• Redundant gas supply is available if there is a problem with one cylinder/regulator
• Cylinders can be easily banded together at the water's edge if a cave requires considerable portage to gain access to the dive

Disadvantages

• Diver needs to monitor both cylinder pressures and be disciplined to breathe the cylinders down roughly equally
• Wearing cylinders on the back limits a divers access to some Australian caves and cave passage