Why the twinset? Safety!
What is the most common failure? A free flow.
What does it do? Throws all your gas away.
What's the solution? Turn the cylinder valve off.
But then I die???? So carry two.
Hence the manifolded twinset as popularised by US cave divers (deep cave diving being so ludicrously suicidal that anybody who lives long enough to bang on about safe kit is clearly onto something).
Two cylinders with two first stages and two second stages. So it isn't too complicated (keep it simple when your brain is full of nitrogen) a joining manifold. When something goes wrong and there isn't an obvious fix FIRST shut the manifold. Now whatever has happened half your gas is safe. If the fault was a pillar valve half your gas will go but if it is a reg (most common fault) you can shut the pillar valve and reopen the manifold and you have all your gas to signal your buddy and get out of there.
Why breath the long hose? Safety!
We all like wreck diving but having your buddy suffer a serious failure might
mean getting him out of somewhere tight and if he has just done a serious OOA
swim it will be silted up big time.
Rule 1: take the reg in my mouth. Don't ask. Don't signal. You are in
trouble and haven't got time to be polite and I am a big boy so I can cope. I
promise that this reg works, that this reg is the right mix for the depth and
thit it is right where you can see it. I have another hung round my neck. Get
some of my gas into your lungs and live. I can find my secondary in a total
blackout in a moment. When you have got yourself back together we can make our
way out/up. You go first if it is narrow, because the hose is long you can swim
through the passage/bulkhead and because I'm not pulling you you don't get the
reg pulled out of your mouth. We are not pinned face to face so we can both
work together and do a normal ascent and because we have lots of gas we can do
any deco owed.
Why the wing? Safety!
In the twinset there is 6 kilos of gas. In the stage bottle or accelerated deco
bottle is another 2 kilos. If I am weighted to complete a stop on empty (I may
have a buddy breathing all my spare) I need to start 8 Kilos negative. If I hit
the bottom of the shot and my neck seal goes suddenly I can lose 10(?)kilos.
Now I have to ascend 16 kilos negative and then pin myself on the surface while
my buddy dekits me (I am not going to climb a ladder with full kit with many
kilos of water in the suit while I am ice cold). So I need, say 18-20 kilos of
lift. That's not a stab that's a wing. If something serious fails the object is
to end up back in the boat saying "that was a crap dive".