I'm antisocial with minimal diving instruction, so my opinions should be taken with a grain of salt. I have a thinly veiled contempt for the false security offered in the current buddy system, and the blind following it draws.
I've been diving solo for a while. Manifolded doubles and a pony give me the appropriate amount of redundancy given the constraints. I don't mind buddy diving, but I still feel a competent buddy should be self sufficient (that does not mean that if you aren't self sufficient your not competent).
Octopuses provide virtually no redundancy benefits, they seem more a buddy diving convenience for those rare times when you need to share air. For yourself, malfunctioning downstream regulators will still provide air in the event of a failure, and there's only one thing to do when that happens (ascend, right now!!!), so the benefits to you of a backup second stage without a backup first stage or isolating valve are minimal (but comforting). You still must rely on your buddy for essential redundancy.
Ponies are a redundant air supply. You could just go k-valve and pony, but that's not very buddy friendly: y-valve (h-valve) and two complete regulator systems on your main tank provide you with regulator redundancy and a more conventional "octopus-like" solution for your buddy (just hand off the long hose on your main tank, slip the extra reg around your neck into your mouth, and start your ascent). Besides being a more buddy friendly AND self sufficient solution, h-valves also provide you with the ability to isolate a faulty regulator. With the leak eliminated, and a pony bottle slung across your chest, you can ascend without urgency.
The pony is there for "your" OOA situations (the only OOA situation I regularly fear is getting caught in something (rope, fishing line, ...) and then taking too long to get free, especially near the end of a dive). I think some dive instruction systems advocate using the pony as the primary buddy breathing solution (when present), but I can't imagine that being all that necessary in my open water diving environments (the regulator in my mouth is the best solution for my OOA buddy imho, no need to switch over to the pony after stabalizing OOA buddy when we would be ascending immediately anyhow). In cases where it would be more convenient, I think they would reveal themselves and I would hand off the pony bottle.
Keep the pony second stage attached to the pony, not around your neck. Sling the pony bottle in front, diagonally across your chest (under you when horizontal) and you will know if it ever starts leaking. You should keep your pony setup as a self contained as possible with little integration into your rig other than the attachment points for the clips: be able to easily hand it off and be able to easily swim away with it (you might need to leave your rig behind).
My rig layout results in a vertical stack of second stages: one in my mouth (the long hose), one hanging from tubing around my neck that sits just below the one in my mouth, and the one strapped near the top of my pony bottle just under the one around my neck. I have no difficulty locating or deploying any of those regulators (one's in my mouth, ones around my neck, the other is right there where I can see it (right next to the pony tank gauge that I can also always see just by looking down)), and I check all three second stages by breathing off of them at depth at least once during the early part of my dives.
I do not feel my 30cf pony bottle is a nuisance (well, barely, and only above water and while entering/exiting the water). Self sufficiency is a key element in being a good buddy. And no, I have never had to use my pony bottle. I also have never had a regulator fail other than when first turning on the air, and that was a long time ago (before I started servicing my own equipment). I still won't being leaving any of that to chance.
No need for an "octopus" when you have an extra "regulator".
There are a few ways to skin a cat. I myself dive with a LP98 and a 30ft pony. I do not dive with an octo. I have my pony mounted on my main tank with the reg around my neck as a backup. My diving profiles are 125 feet and shallower, most being in the 80-90 ft range where 30 cuft of gas can get you to the surface with a safty stop as long as you are not in a deco dive which I really dont ever do. I dive off Jersey at least once sometimes twice a week and all the boats require a redundent air source. If there is someone who has an OA emergency they will get my primary and we will accend immediatly. A buddy is for equipment malfunction and entanglement. Not to share air.. Keep an eye on the damn gauges. There is absolutely no excuses for running out of air short of Narcosis and severe entanglement. .02 deposited
