So who’s ever brought up their second stage in a lobster bag?

[MaxBottomtime]

OW scuba gear is designed simply and reliable. Maintain it, ensure that any repairs or modifications are correct before getting in the water and odds of a failure are extremely low. Forgetting to tighten a second stage is the opposite of that. Rebreathers are a separate issue. The rebreather accidents I've read about have been due to user error. A diver on open circuit does not need to rely on batteries, computers, and O2 sensors to complete a dive. I would likely have a bailout bottle with a rebreather.

 

[Angelo Farina]

Your solution is another one - I assume you are talking about an H-valve. I personally think that a set of small backmounted doubles is safer and easier to manipulate in case of an emergency.

Yes, an H-valve, or, better, an Y-valve providing no O-ring or other point of failure and two independent air paths (with two separate dip pipes).
A compact double is also very good, provided that the manifold allows for separating the tanks by means of a third valve.
But this setup, albeit very practical, has more possible points of failures than a single with an Y-valve. Considering the benefits and the drawbacks, I evaluate that both solutions are substantially the same in terms of safety. What I consider truly unsafe, even for very shallow depth, is a single with just one valve and an octopus. As in all places were I did dive all singles were always equipped with twin valves, why the hell people insist using an octopus instead of two separate regs? Just for saving money, or weight???
And why do instructors teach students using an octopus, when we should teach that it is a less-than-safe solution?

 

[rsingler]

Yes, an H-valve, or, better, an Y-valve providing no O-ring or other point of failure and two independent air paths (with two separate dip pipes).
A compact double is also very good, provided that the manifold allows for separating the tanks by means of a third valve.
But this setup, albeit very practical, has more possible points of failures than a single with an Y-valve. Considering the benefits and the drawbacks, I evaluate that both solutions are substantially the same in terms of safety. What I consider truly unsafe, even for very shallow depth, is a single with just one valve and an octopus. As in all places were I did dive all singles were always equipped with twin valves, why the hell people insist using an octopus instead of two separate regs? Just for saving money, or weight???
And why do instructors teach students using an octopus, when we should teach that it is a less-than-safe solution?

@Angelo Farina has an excellent point. It's funny how set we get in our ways. Y-valves are very uncommon on our side of the pond, but perhaps they shouldn't be.
On the other hand, given American preference for "low price" over almost anything else, two separate firsts might be a hard sell.
But for single tank diving, I'm giving serious thought to the notion of adding a Y-valve, since there is no shortage of first stages lying around the shop...
I think it may come down to risk/reward. Does the added possibility of a first stage failure outweigh the benefit of having a solid solution to a failure like the one described so nicely by @uncfnp ?

 

[lowviz]

But this setup, albeit very practical, has more possible points of failures than a single with an Y-valve. ...//... What I consider truly unsafe, even for very shallow depth, is a single with just one valve and an octopus. As in all places were I did dive all singles were always equipped with twin valves, why the hell people insist using an octopus instead of two separate regs? Just for saving money, or weight???

I change the hell out of my config almost constantly, part of my fun of diving. Some configs that seem good on the surface end up totally sucking underwater. The latest case in point was my unmatched tiny seel doubles that I took to an ITT course. Diving them under the 'pressure' of a course was incredibly valuable TO ME. My point is, that when you keep messing with your gear config, you find that you become able to compare and contrast.

...//... -why the hell people insist using an octopus instead of two separate regs? Just for saving money, or weight???

I share your concerns, but not your indignation over PARTIAL redundancy.

I have come to appreciate the value of true redundancy. I see the value of sidemount, did it and got the shirt, but I'm much more of a tiny doubles and pony guy. Maybe has something to do with being an old (and still alive) solo diver...

Anyway, I have an H-valve on an HP-120. Probably sell it, doesn't get out much. What it DOES do is keep things simple when switching from singles to doubles. But it doesn't provide redundancy on what I really want, which is something to breathe.

A properly sized and mounted pony is the slickest thing out there. I can drop everything in a hot rush and grab my pony as the very last horse out of Dodge.

 

[Marie13]

I change the hell out of my config almost constantly, part of my fun of diving. Some configs that seem good on the surface end up totally sucking underwater. The latest case in point was my unmatched tiny seel doubles that I took to an ITT course. Diving them under the 'pressure' of a course was incredibly valuable TO ME. My point is, that when you keep messing with your gear config, you find that you become able to compare and contrast.

I share your concerns, but not your indignation over PARTIAL redundancy.

I have come to appreciate the value of true redundancy. I see the value of sidemount, did it and got the shirt, but I'm much more of a tiny doubles and pony guy. Maybe has something to do with being an old (and still alive) solo diver...

Anyway, I have an H-valve on an HP-120. Probably sell it, doesn't get out much. What it DOES do is keep things simple when switching from singles to doubles. But it doesn't provide redundancy on what I really want, which is something to breathe.

A properly sized and mounted pony is the slickest thing out there. I can drop everything in a hot rush and grab my pony as the very last horse out of Dodge.

Mr. Tinkerer you are.

Pony is the easiest thing to add to your standard setup. Gotta have a BC that can sling your pony. Otherwise, it can be a hot mess (as I’ve observed at the quarry or on the boat).

 

[rsingler]

Nobody, or almost nobody carries Y-valves. This is all I found that's new:

In terms of points of failure, a Y is probably preferable to an H.

But given the vendor supply issues and lack of redundant gas, I think @lowviz is right. I'll just stick to my ponies. 13CF for shallow, 19CF for deeper. Or doubles.

 

[Angelo Farina]

Although I own a 3.5-liters pony, aluminium tank, with single valve, I used it rarely and only for deep dives with deco, where the deco tanks waiting for you at 6m are not considered safe enough and you want a truly redundant and independent air supply.
Being old, now, I prefer to dive shallow, no deco, and to be as much lightweight and streamlined as possible. Hence I use a single, narrow tank, with a very light rear-inflating jacket, nothing hanging around, a thin 3mm wet suit (apnea style, low friction), with long freediving fins. So I can move really fast and with minimum drag, almost as a freediver.
A pony would be impacting severely on such a low-drag approach...
However, as all single tanks, even of just 12 liters, here are invariantly equipped with twin valves (mostly H, sometimes Y, which I prefer), I always use two first stages. Simply, it would take time to reconfigure to octopus, and I do not see the point doing that. It will be less safe...
I was using an octopus only when I was working as DM and instructor, so to have an octopus plus a second independent reg. So I had one primary and two secondary second stages, being able to supply air to two students simultaneously, if needed.
For that task, the secondary of the octopus was mounted on my left shoulder with a very long hose: that was the second stage "for others", not to be used by me...

 

[lowviz]

I'll just stick to my ponies. 13CF for shallow, 19CF for deeper. Or doubles.

Or both.

My pony IS my standard and unchanging, highly maintained, base gear that I never use. All the rest of the stuff I strap on for the dive is always under consideration/suspicion.

with long freediving fins. So I can move really fast and with minimum drag, almost as a freediver.
A pony would be impacting severely on such a low-drag approach...

Backmount it in your slipstream?

 

[Angelo Farina]

Or both.

My pony IS my standard and unchanging, highly maintained, base gear that I never use. All the rest of the stuff I strap on for the dive is always under consideration/suspicion.

Backmount it in your slipstream?

Yes, I usually employ a single BM and I leave the pony at home.
Tested sidemount, but using just a single it becomes strongly asymmetrical, and I do not swim well. Furthermore the rear-inflating jacket does not fit well with a single on one side.
I keep the bottle behind me when moving slowly. But for maximum speed I bring the bottle in front of me. When young I was an athlet (and an instructor) of "underwater speed" (aka "velosub"), using a monofin and keeping the small bottle in front.
At the time, we were trained also wearing and unwearing the rear-mounted tank (even the doubles) "from above the head", both on the boat and underwater. So when needed we were used to open the velcro strap and bring the bottle(s) in front, for going through small passages, etc...
This way:

 

[lowviz]

But for maximum speed I bring the bottle in front of me.

Envious of that dolphin kick. I could never sustain that style for very long. But to the OP's point, having a small bottle is useful should one choose to exit stage left...