The use of a single cylinder may provide enough gas to allow an ascent from recreational depths, but the volume of a single cylinder is simply not enough to allow an ascent plus decompression stops from greater depth. One tank means no redundancy and the words redundancy and technical diving go hand in hand (If you don’t need it, leave it. If you do, take 2). The use of twin cylinders/doubles or twinset is a way of providing this redundancy.
Sizes
A twinset is usually made up of two of the same sized cylinders with a regulator connected to each
cylinder. Twinsets made up of 12L, 15L, or even 18L cylinders are available but, for the majority of technical divers, twin 12L cylinders provide a good balance of weight and gas volumes. 10L twinsets aren’t uncommon with smaller ladies who are ‘light on fuel’ and don’t want to carry the weight of bigger cylinders.
Independent
Twinsets can be configured as independent or manifolded.
Independent cylinders provide complete redundancy, as there is no link between the two cylinders. Thus, if one cylinder has a problem, the other is completely independent. However, as the two cylinders are independent, the diver has to switch from one to the other in order to balance the gas usage in the two cylinders (just like Sidemount diving). Whilst switching regulators should be easily within the skill set of a technical diver and should be a routine action, it can sometimes be forgotten when the diver is in the middle of a problem. We do NOT recommend independent cylinders.
Manifold
The other (better, more widely acceptable) option is to manifold the two cylinders together. This involves connecting the two cylinders at the valves by means of a manifold. On an isolation manifold the left and right cylinder valves allow the corresponding regulator to be shut off, leaving the entire gas supply (cylinder) to be used through the remaining regulator. The central valve, 
separates the tanks into two independent systems, each with its own first-stage and second-stage regulators, which can prevent a failure in one half of the system from losing the entire gas supply. This also has the benefit that the gas from both cylinders can be accessed from the primary regulator. The disadvantage is that, in the case of a problem, the diver must shut down the problem regulator, or isolate the two cylinders by means of the manifold, otherwise the gas from both cylinders will be lost. It is essential that a diver with a manifolded twinset can carry out a ‘shutdown’ to prevent the complete loss of their gas. As a twinset diver you need to be able to do shutdowns with your eyes closed (literally in some situations).
Final thoughts
10 years ago most tech divers, especially cave divers would be seen with twinsets. This has changed over time. Rebreathers are almost commonplace now and Sidemount has made a huge entrance in the market.Doubles are quite expensive to put together and a bit heavy to carry outside of the water. But if your back can handle a little bit of extra weight, you will not be disappointed. The added weight works for you when diving a drysuit. The balance is brilliant. Streamlining easy (everything has its place). No HAVING to change regs constantly. And a big plus – modular dive systems. Upgrade parts, not entire setups.