Single tank, H-valves or doubles?
When we do recreational no decompression diving a single tank is the weapon of choice.
For anything deeper than 100ft / 30m or anything involving decompression or an ovearhead environment like wreck penetration, caves or similar the doubles are the right choice.
The reason is simple.
If we do a dive in the ocean and we experience a malfunction with our equipment and can't breath we can signal our buddy and share gas with him.
Obviously we want to terminate the dive but what if he also has problem now?
Well, we could both do a swimming ascent straight to the surface. That gives us the ability to handle two severe problems without getting into real trouble.
When we venture into an overhead environment or we have mandatory decompression to complete we have to stay down and our option to go to the surface has been removed.
Now we don't have any way to survive two severe equipment problems. So we need to carry more equipment - what we call redundant equipment.
A set of doubles has two connected but independent regulators consisting of one first stage and one second stage each. If one fails we switch to the other.
If that one fails too we start sharing gas with our buddy.
Why not just a H-valve and two first stages on a single tank?
That may seem like a good solution but it's not really for a couple of reasons.
Deeper dives, decompression and overhead are complex dives and since we don't have the option of going to the surface we need to handle the problem under water.
To be able to do that we need to carry more gas then we intend to use - reserve gas. If we are deeper 5 minutes worth of gas is a whole lot more than it would be shallow.
So these dives require a bigger safety margin and a single tank is simply not big enough to carry an adequate amount of reserve gas.
Another drawback to using two first stages on a single tank is complexity.
You need to be as familiar with your configuration as a double tank diver but also you need to be able to reach back and operate your valves
because if you can't shut it down you have gained nothing. H-valves and Y-valves are often difficult to reach and the configuration is not streamlined and neat
because you have to much regulator and not enough room.
Another reason why some want to use an H-valve is so that they can use the same setup for their doubles.
Besides the very busy configuration you also get the wrong hose lengths because these have to
fit you doubles where the first stages are located on each tank and not in the middle. So while it might work it is not optimal.
What to do instead?
Well, I suggest getting a dedicated first stage for single tank diving with the right hose lengths.
It's not a major investment and if don't use it often you could skimp on getting second stages and just move those between you double regs and your single reg.
You also have the advantage of being able to use any tank on your vacation since you have it setup already.
And most important you don't have to deal with the unnecessary complexity of having two first stages that is of no gain because any dive requiring two first stages also requires the added gas of a set of doubles.
So in summary use a single tank with one first stage for dives down to 100ft / 30m not requiring any decompression and not venturing into any kind of overhead.
Use a set of double tanks for the rest and skip the H-valves all together
Dive safe and have fun!