Matt, diving with an attentive buddy should take care of your problems. But you are right; many people post stories of buddies who didn't perform as they hoped. And there is no guarantee that your "instabuddy" will have practiced ANY skills since OW -- we just taught a rescue class to two guys who had been dive buddies for several years, but who admitted they hadn't done a practice air-share since their original class. And they didn't do all that well when asked it do it from their rusty state!
You can minimize the problem of buddy separation by a careful pre-dive discussion, where you emphasize that staying together is one of your top priorities. But you will still need someone with the SKILLS to stay together, which in the kind of water where you have been diving, is not all that difficult. But in lower viz, it can be more of a thing.
Having your own redundancy is not a bad thing. (Cue the hissing as a huge SB audience that knows I'm a DIR diver inhales through its teeth.) Double tanks are great, but not available for rental in a lot of resort-type locations, and you have to learn to dive them to make them an advantage; without training and practice, they're actually worse than single tanks in terms of failure points. A simple, slung pony bottle has a lot going for it -- it's easy to carry, it's where you can see it, and it can even be handed off to another diver if need be. The biggest issue with pony bottles is that people underestimate the volume they need. You have to figure that if you need the pony, you're likely to be breathing a lot faster than you normally would. Since an Al40 is pretty comfortable and easy to sling (assuming your BC has the needed attachment points) it's hard to make an argument for something much, much smaller. I have not been happy with the balance of anyone I've dived with who mounts the pony bottle with brackets to the primary tank. That's also a bad place to have it, because you can't reach the valve on it, and you can't see the regulator attachment or first stage to troubleshoot problems. Watch Dumpster Diver's recent video of his son having to deal with the problem, when his pony bottle developed a massive leak.
It is GOOD that you are thinking of the what-ifs. It is good that you are considering various approaches. The approach I use to the problem you posited, of catastrophic gas failure, is to dive with attentive, trained buddies who practice skills regularly. On the occasions when I don't do that, I dive doubles, and I know how to use them. (I often try to find a third team member who's reliable, too.) In your case, obtaining an appropriately sized pony bottle and learning to be facile in its handling and use may be the best answer, given the pattern of diving that you are likely to do. Carrying a pony does NOT negate the other benefits of diving in a buddy team, though.
If you do this, make sure you practice a lot before really counting on it. 
