Wife & I just went through a refresher course.  We were both certified back in the day, but I had very few dives while on active duty and she had even less when she was going to school.  We're both getting back into the sport and just go cert'd for OW through SSI.  I've ordered my setup of what I want, she's ordering hers on Sunday.  We're both going to get our Stress & Rescue certs and several other courses... we've got a few lakes that we want to start hitting and a few instructors from the dive shop go out every other weekend or so.

One of the things I'm tossing around is the idea of either having an AIR2 on my BC and then a small pony tank, or just having an octopus.  I'm leaning towards getting an AIR2 and a pony tank for a someone in distress situation as I'm not too keen on most people having access to my air and sucking it down while they're panicking.  Plus, if they aren't controlling their buoyancy and try to emergency ascent, I don't want to go along for the ride.  What are y'alls thoughts on this?  I know I can't be the only one that has or has had this question in their own mind before.  Thanks in advance y'all.  
-SleeperShooter

As it stands today pony bottles really don't make a lot of sense anymore. They're sort of a pain in the butt to mount/sling; not unwieldily but mildly annoying. If you travel you either have to leave it at home or drain it and take off the valve to get it into your luggage; that means the bottle is at an increased risk of contamination and it's extra weight that you may have to pay for. Then there's the size conundrum. If you go too big you start leaving it at home and too small means that it really doesn't offer much advantage.

If you're going to go thru the hassle and expense of going the pony bottle route you might as well go side mount. By the time you're done with the expense of an additional regulator, tank, and possible bracket you're into the expense side of the equation for side mount sans training. Side mount offers a couple of advantages:

-Side mount travels very easily.
-You can pretty much side mount any tanks you can find. If you can get two AL80's from Joe's dive shack on the beach you can side mount.
-You have easy access to all your controls, much better than back mount.
-You've doubled your gas supply. Gas is a lot like sex, you can never have too much.

Now I'll play devil's advocate with my own argument. For most recreational divers a single AL80 is more than enough. The reality is that open circuit equipment is pretty darn reliable and if you're worried about running out of air just get a bigger tank. Personally, I wouldn't go the air source route, I think they're too restricting in terms of head movement.

If it were me for open water I'd run a 5 foot hose on my primary regulator and run my secondary on a 24 inch hose and bungee it around my neck. Now you simply donate your primary when you have to share air and switch to you secondary necklaced around your neck.

YMMV

If you think yer gonna spend a lot of time on the bottom, pass on the air2.  I watched one go into free flow after it scooped up a bunch of sand from dragging along the bottom...

If they're truly panicking:

A) They're probably not going to accept air from any source.

B) You're not going to want to maintain face-to-face contact to try to cram a regulator into their mouth.

If your buddy is having an out-of-air situation, and not panicking, then an octopus would be sufficient.

Watch the video in this thread:  Link

I don't plan on swimming in sand and dragging my gear, but I guess that could happen.  From what the pictures show, it looks like the Hydros comes with the AIR2 on it already, so that may have made the decision for me already.

Squatch,
 Thanks for chiming in.  I did watch that video and that's what got me thinking about secondary removable air source.  From one Eagle Scout to another, I tend to rather be a little over-prepared than not.  

Panicking diver:

Yeah I also saw that one.  I spent a few "minutes" the other night on YT watching some various videos.  You know how that goes and starts..... LOL.

Just remember, that panicking divers are not rational and can make you the victim of a diving accident as well.  Best preparation to avoid panic is to dive within the level of the least experienced diver in your buddy team and to have properly maintained equipment (along with pre-dive checks), IMHO.

I am personally against a pony bottle except for your own use.  

I have seen them used once on a panicked diver.

In this case a new diver and instructor and myself were taking a deep dive in a local quarry.  At the bottom the other diver ran out of air at 120'.  Instructor gave him a pony bottle.  It was gone in less than 1 minute.  Lucky for the panicked diver the intructor was wearing doubles. That octo  was the only reason he made it to the surface.

Valid point.  Mission drives gear.  Murphy's law states that failure will happen at the worst time, and of course that's going to be a depth.

I have a 19 pony, on the rare occasion I dive alone I bring it.  When we dive murky water metal detecting we wear them as well...

Nothing replaces a competent dive buddy though...

When I solo dive, I carry a 19 cu.ft. pony AND a Spare Air in my BC pocket.

When I dive with a buddy or on a commercial boat, I have an octo and the Spare Air (which is for MY use only).

I have a Air2, several different pony bottles (deco) and  octopus. It depends on my dives what I prefer to setup and use.  However based on basic diving I would recommend the air2 and here is why.

1- If you have a free flow on the air2 like dragging it in the sand you can disconnect the LP hose. If your octo does that you cant.
2- It streamlines your gear and you have less stuff dragging around.
3- It is easy to use.