Double tanks require the use of a manifold and bands and bolt kit. The manifold valve system replaces the standard K valves that probably came with the tank. The manifold combines the air supply from the two tanks allowing you to breathe from both tanks at the same time. A manifold system requires the use of two regulators so that you can breathe from both tanks from either regulator. Most manifolds come with an isolation valve in the center of the manifold, which is used to isolate the air between the two tanks. If a burst disk on the manifold blew for one tank, and you needed to stop the air loss from the other tank, you could close the isolator valve to stop the air loss. A burst disk failing is very uncommon, which causes controversy on whether an isolator valve is needed or not. Regardless, technical divers typically use manifolds that have isolator valves.
When shopping for a manifold, look for one that allows you to adjust the distance between the tanks. Don’t buy a “yoke” style manifold; only buy and use a DIN style manifold. Yoke regulator/valve system o-rings fail too often. Look for a manifold that has a barrel-style o-ring system, not a face-seal type. Barrel-style systems allow the manifold’s center section to move so that if it ever came loose the o-ring wouldn’t extrude and cause a leak.
Cost: $200 to $300
Y or H valve
Y valves or H valves provide two outlets where two independent regulators can be attached (even though there is one bottle). Even if a regulator freezes, this type of valve allows a separate outlet so that there is no loss of air.
Cost: $75 to $150
Bands and bolt kit for double cylinders
A bands and bolt kit is required to hold double tanks together and provide you a place to mount your wing BC and backplate and harness setup. The bands should be welded stainless steel and should be the correct size for you tanks. The bolts should also be stainless steel and the right length for you tanks. Each manufacturer provides the specifications for their band and bolt kits and indicated what size tanks the kits work with.
Bands: $100 to $150
Bolt kit: $15 to $20