"Octo" is an interesting thing... it's short for "octopus" which some people use to refer to the safety second stage, and some use to refer to the entire regulator set, due to the fact that many open water setups look like an octopus with 4 hoses dangling. I bought a regulator recently from a guy who called it all a "spider" for the same reason.
Then, when you refer to the safety second stage as an "octo" and the one you use as a "primary" then it does get tricky. Some say "well I use two primaries, and one primary is the safety second."
What they typically mean is, that they use two adjustable second stages.
However, whether adjustable or not, it's still a second stage. It seems like a lot of confusion here is stemming from the fact that the OP wants a second stage that is non adjustable, and goes in line with the lp inflator hose for his wing/bc.
I am not a fan of those style regulators, and would tend to want to put the safety second stage around my neck on a bungee, "tech style." There, you can use an adjustable or non adjustable second stage. I have a mishmash of second stages on a lot of regulator sets...for sidemount, I've got a Hog on one side and a non adjustable Scubapro reg, sold as an "octo", on the other. But on my o2, I've got a nice, expensive Scubapro X650, adjustable regulator. Why? Some think it's so I can relax and not have to worry about the reg breathing hard at all on deco, but in reality, I have a friend who really didn't like the "darth vader" style of the x650, so I loaned her a Hog and took the x650 to use on my gear. I'm not the biggest fan because it's kind of ugly, and I sometimes dive with photographer friends, so I put it on a bottle that won't get photographed much.
In short, my advice is to have the Hog as your "octo" if you are going to have a Hog as your "primary." Its simple, high performance, low cost, self servicable, and all around good performer. If you want to distinguish it as the "octo" then buy the yellow rubber face for it.