Hi there!
I've been down this path myself and I eventually found answers that I'm currently very happy with. My answer as to whether or not to get the pony bottle is no.
My buddy is my redundant system. What we need are good protocols and good training, not slapped on equipment to circumvent the former two.
Divers run out of air because of poor gas management doctrine, rusty skills, sub-optimal equipment, poor situational awareness, poor team positioning and in very rare cases, equipment malfunction. I'll briefly go through each.
Poor gas management doctrine - E.g. I was taught to start my ascend at 50 bar. But is this good doctrine? Ascending at 50 bar from 30m requires very different amounts of gas from ascending from 10m. Is ascending from 30m with 50bar enough to share with my buddy if his regulator starts freeflowing uncontrollably at that point? Is there enough air to do your safety stops properly so you don't get bent? Clearly the answer is no. This is not a good doctrine. Much better is the concept of Rock Bottom taught by some agencies such as GUE/UTD.
Rusty skills - when was the last time me & my buddy practice our air share drills? Are we able to still do them under stress quickly and smoothly? Have we done it so many times its ingrained in muscle memory? It's a good idea to devote half a dive out of every dive trip to just go through the drills. Doesn't take a lot of time.
Sub-optimal equipment - it's common to see the backup octopus dragging on the sandy seabed on many divers. When it's needed and the OOA diver puts it in his mouth, he's going to choke. I think you already have this settled by using the longhose/necklace.
Poor situational awareness - Not checking the gauge ever 5-10mins, not checking on buddy every minute or less, devoting too much attention to equipment or photography or fighting to control buoyancy, all these takes away a divers attention to other needed things.
Poor team positioning - Not descending together, not swimming together, not ascending together, buddy too far when an emergency occurs
Equipment malfunction - With good protocols & training, it'll take a very rare event of having both diver's equipment malfunction at the same time to create a life-threatening emergency. Mitigate it by not servicing your regs at the same time for e.g., in case the servicing job was not done properly.
Check out GUE Fundamentals, where all this is covered, no point trying to reinvent the wheel.