On most dives, I sling a 19cf Al pony, but sometimes, I may dive with a 30cf Al pony (and occasionally, I'll even sling an AL80, just for fun). Regardless, I always sling it.
If you're not familiar with "slinging" a bottle, it involves making a little "harness" out of some plain old line ("rope"), two clips, and a screw-type clamp (i.e. big "hose clamp"), and usually a little bit of bungee to hold the hose(s). It took me all of a few minutes to rig each of my ponies. (We can post links to instructions if you want, but I don't have the link handy in the middle of the night here. :biggrin
Anyway, the pony ends up with two clips (bolt snaps, trigger snaps, whatever), one clips to my shoulder D-ring, and the other clips to a D-ring on my hip on the same side. That leaves the valve and first stage nestled right in front of my shoulder, where I can see it (and where I can be sure nothing is going to hit it). I can turn the valve on and off with either hand. Unless you're lying down on the bottom, a bottle slung like this really doesn't get in your way. You very quickly get completely comfortable with it, and having the potentially leaky bits in front of me supports my confidence in the readiness of my pony.
My ponies used to sport plain yoke valves, but I have since moved to DIN for the ponies and their reg. The absence of a yoke screw sticking toward me was the primary reason I did that. I would occasionally get something hung up on it, so I just went the easy way and eliminated the problem. (I can still use yoke if I need to -- they're convertible valves, and I have a yoke adapter for my pony reg.)
As for second stages, I have two second stages on my primary regulator. The "octo" there is my preferred method of air sharing should I be diving with a buddy and need to share. Additionally, it can prove quite useful in the event of, say, a bout of nausea resulting in fish-feeding. Being able to rinse a second stage without going to the pony can be... useful. If I didn't have the additional second stage, the dive would be over -- I have an Draconian rule that if I go to the pony (except to check it at the beginning of the dive), the dive is as over as if I thumbed it myself. (If that were negotiable, I might end up thinking of the pony as usable air to extend the dive, and *that* is the cardinal sin that pony divers *must* avoid.)
On the pony, I have but one second stage. The pony I dive on any particular dive is sized based on *my* gas consumption to get *me* safely to the surface in the event of a serious failure in my gas supply. It is not there to share (which is not to say I wouldn't use it to attempt to save someone's life, but that would be so far outside the plan that I'd consider it beyond reason to gear up for it). The pony reg, then, has a first stage, a single second stage, and (in my case) a full-size pressure gauge. (I also have a backup button-style "pony gauge", but I prefer the readability of the normal SPG. I can use the pony gauge and swap the SPG out to my primary reg if my primary SPG ever fails on a trip, and I've actually done that for a friend who blew an HP hose.)
Anyway, that's a little of how I dive with my pony (which I've had since my first logbook had single digits), and if any of this was rambling, please allow me a little leeway, as I'm up through the night for the second day in a row. :biggrin: (J. is now on the ground back in Japan, but I have to stay awake a while longer before she can call.)