Mr Mares:
Divers using twin tanks commonly use a system in which the tanks are joined by a manifold. Essentially, the manifold is a pipe threaded into the left and right tank valves that connects the two tanks and allows gas to pass between them when the isolator valve is open.
Manifolded twins have three valves. The first is on the right tank and is called the right post. The second is on the left tank and is called the left post. The third is the isolator described above. It is disposed in the middle of the manifold.
Each post has a first stage that is connected to a single second stage. There are other things attached to the first stages, but they are irrelevant for the purpose of this discussion.
The valve on each post turns off the gas flowing to the first stage to which it is connected. However, turning off a tank valve does not turn off the manifold. In other words, if I turn off my right post, I will lose the ability to breathe from my primary regulator because the primary is connected to the right post. However, I can still breathe the gas in the right tank because the gas will flow across the manifold to the first stage connected to my left post, which is connected to my backup regulator.
Turning off the isolator shuts down the manifold. When the isolator is closed, I can only breathe from the tank to which the regulator I'm breathing from is connected.
Accordingly, if I have a problem with a first or second stage regulator, I can shut down the reg and still access all of my gas. If I have a problem with the tank itself, such has an extruded tank o-ring or some other major disaster, shutting down the post will not solve the problem. At that point, I need to isolate. If I don't, I'll lose all of my gas.
Hope that this helps.
With regard to the original question, imho, if you need a redundant air source, go for doubles. The reason I believe this is that the pony is, by definition, a more limited system. I'd rather have my full supply of gas available in as many failure modes as possible instead of relying upon a smaller supply at a time when I'm facing some type of crisis.
Unless I'm forced to isolate, thereby cutting my gas by half, I know that I'll have enough gas to get back to the exit point of the dive without a problem. In addition, unless I've isolated, I'll have enough gas to share if I need to, although I'll freely admit that air sharing with a right post failure is not fun. However, since all of my buddies are also in doubles, the chance that this will happen is relatively small.
Twins are certainly not necessary for every type of diving. Proper gas management will make sure that you and your buddy always have enough gas ascend safely to the surface while sharing gas. Whether this means returning to the point where you started or whether it means an immediate surface depends on the conditions of the dive.
