Ok so it looks as though I'm going to be the only one to desent from the "Keep the Octo" crowd. My first question is why? You have your primary regulator on your main tank and a secondary regulator on a pony, two fully redundant breathing systems, so why do you need the octo? If you run out of air, switch to your pony and abort the dive. After all, this is the reason you bought the thing isn't it? If your buddy runs out of air pass them the pony and abort the dive. Doing the ascent this way means that you don't have to ascend while holding onto each other to keep from pulling the short hosed reg out of his mouth, much easier. This obviously means that you have to mount the pony in such a way that you can remove it easily if you have to, slinging the bottle is the easiest way to do this. I've mounted a pony perminately on the back tank and I currently sling any stage or pony I carry so I have experiance in both.
Besides the obvious benefit of being able to get rid of one hose, and one potential failure point, from your primary reg it's also a safety issue. 3 second stages hanging in front of you can be a little confusing in a stressful situation. Which one do I breath off of, which is the pony, which is the primary and which is the octo? Not something you want to have to deal with when one of you is out of air. That means that each reg has to be a different type or marked is such a way that there will be no question as which reg is which. How are you going to do that...a different color for each reg? Colors fade to blues and grays at 40 feet or less so that won't work. Maybe a different size or shape...what happens when it's a night dive, maybe just dark water or bad vis or you have heavy gloves on because the water is cold making it hard to identify by feel? I don't think I want to have to try to figure out which reg I need to breath out of if I'm low or out of air, it's dark and I'm fading fast as my last breath was a minute and a half ago. I've have heard a story of an Instructor diving the USS San Diego with doubles and a back mounted pony. The guy mistakenly put the pony reg in his mouth when he splashed and by the time he had reached the bottom and had penetrated the wreck he ran out of air. They found him with his doubles full and an empty pony. More then likely he couldn't figure out why he was out of air with his spg showing full tanks and with three regulators to choose from he panicked and died with more then enough gas to exit the wreck and surface.
KISS (Keep It Simple Stupid). If you don't need it don't bring it, if it's something that could cause a problem get rid of it. IMO you don't need the octo and carrying it in this situation can only lead to trouble. Get rid of it.
Safy diving.