I dive long-hose (7') with a bungied backup.
If I dive with someone who I don't usually dive with, I go over the fact that if they need air they should feel free to help themselves to the regulator that I have in my mouth. I will appreciate it if they ASK for it, and unless they come from behind I'll probably have it in my hand for them before they grab it, but in an emergency that is the one they want - and I fully expect they'll TAKE it if necessary. This is all explained before we get in the water - and I go over where THEIR regs are too.
The backup around my neck is mine. They are welcome to try to take that one if they WANT to do so, it WILL come off if pulled hard enough (the surgical tubing I use for the bungee will break) but its on a very short hose and its probably a bad idea for them to grab THAT one, as it might not reach their mouth.
I also test the backup on every dive - usually on the decent and often at the safety stop. This way I KNOW it works, since I checked it when I started the dive. That's a good thing, considering that its my backup! My backup is slightly-detuned, but not significantly so (R380.) Unfortunately it has a yellow purge - if I could find a black one I'd replace it, but so far I haven't found one... oh well.
I hate traditional octos. If I'm truly about to bug out I'm not going to grope around in someone's "golden triangle" - I'm gonna take the device that I see bubbling, whether you like it or not. In open water the Air2 is a rational choice - I trained with one of those, and it does work. But in any confined space they could be lethal.
If you dive a tradtional octo you ought to make sure YOU can get to it immediately and that YOU know where it is ALWAYS, and that it works, because the odds of someone being "nice" in a true emergency are somewhere between slim and none.