At DIR diving double tanks are most common used. The tank size depends on the planned dive but some sizes are preferd due to their trim characteristics. Most common are double 12 liters, double 18´s and double 20´s. For diving fresh water steel tanks are the choice, for saltwater aluminium tanks are used e.g. double 80 cuft.
The tanks itself are held together by stainless steel clamps. This avoids, opposite to flexible connections, stress to the manifold. The clamps are held by thread bolts which are used as well for the connection of the backplate. No boots are used for the tanks. Sometimes “doubleboots” are used to substitute the lower clamp. This has to be avoided for several reasons. These boots can´t give the needed solid connection like a stainless steel clamp. An on top of that you will get rust underneath the boots which can´t be seen and damages your tanks.
The isolator manifold is during the normal diving always in the open position. So you can use the whole volume of your tanks. In case of a damage of one of the valves you can safe half of your gas by closing the isolator manifold. In case you´re having a damaged regulator of a freeflow, you just have to close the according valve and have still access to the total volume of your tanks.
The valves itself have a DIN-connection. This ensures a stabile connection between the valve and the first stage of the regulator (e.g. if you hit the ceiling of a cave or a part of a wreck). With the yoke version the connection is not a stabile and therefore has to be avoided. Some use 300bar versions of valve and regulator to benefit from the longer thread.
The isolator manifold has to have barrel-style O-rings and not O-rings facing the valve side. If the distance between the tanks respectively the valves gets wider, the manifold with the O-rings facing the valve would leak. It could also happen that the manifold becomes leak if you hit a cave ceiling with the isolator-valve and the manifold gets turned. With barrel style O-rings this is possilbe without any problem. To avoid to break your isolator in that situation, it shouldn´t be fixed but be turnable. wird, sollte sich die Brücke drehen lassen.
For the valves are rubber knobs used. They have the advantage that they won´t break as easy as plastic ones if you hit something (especially at cold). If you brake your valve knob, it is impossible to close that valve. On top the rubber knobs are more easy to operate with gloves.