It's surprising to see support for separate cylinders and regs all on this board...I was starting to think I was the only one here who dove this configuration.
I have to agree with the "gas management" problem being complete crap. A pointed out above, it does technically increase task loading and requires switching regs, but then you have to be realistic and understand that all divers should be constantly aware of gas supply and that all that is added is switching a reg near the mid point of the dive and again toward the end of the dive. If you can't handle that kind of really minor task loading you really should not go in the water.
A manifold does potentially allow you to access all of your air in the event one reg fails but it also requires you be able to shut the offending reg off in a timely fashion or else you risk losing all your remaining air in short order. (you'd be amazed how fast a modern high performance reg will dump air in a full fledged free flow.) An isolator valve adds another level of saftey in being able to isolate your tanks to what is in effect separate tanks and regs should a valve, neck o-ring or burst disc fail. But at the same time, it also adds another failure point with the result that tech divers as a whole can't completely agree on whether an isolator valve is a good idea.
In my opinion, an advantage of separate cylinders and regs is that with proper gas management no action is required to ensure you have adequate gas to reach the surface from any point in the dive. On the negative side you potentially lose the availablility of some gas when a failure occurs, but with proper management, it's gas you don't need. On the plus side, you get the advantages of an isolator valve without the additonal failure point or the absolute need to shut off a valve following a failure. You also get the advantage of being able to dive a familiar configuration when traveling, and I have never found a shop (outside of one doing tech training) that rents manifolded doubles.
And realistically if you properly fill and maintain your tanks and valves are not scootering through caves, the most likely failure you will experience will either be a minor slow leak that most divers would not even notice or it will be a frozen and violently free flowing first stage, that can be resolved by shutting the tank valve for a minute or two with either a manifold or separate tanks and regs.
You also don't see many people bashing cave divers who side mount their tanks which is functionally the same as a separate cylinder and reg independent doubles setup from an air management standpoint. It's just a matter of the advantages of a particular configuration out weighing the disadvantages in your specific type of diving. A lot of divers are opposed to independent doubles based solely on the philospohy of their training agency rather than on an open minded assessment of whether they may be approriate for a specific diving situation.
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