I use two pony systems - pony attached to main tank and pony on a sling. If on the sling, I can easily reach down and turn on. If it's on my main tank, I can reach down and turn it on, but with some difficulty. Difficulty is not what I want in a emergency/out of air situation - so I leave the air on.
I have two sizes of pony - 13 & 19 cu feet. For dives 50 ft and less I use the 13 cu ft, for deeper dives I use 19 cu feet. Lately, I've just been using my 19 cf exclusively - deep and shallow dives. My tank attachment is configured with a tank strap that easily slides onto (and tightens with an regular tank fastener) my main tank and therefore can be readily exchanged between tanks. Obviously a sling can be slung and re-slung - however, I don't use the sling so often as I do a lot of hunting/crabbing and it gets in the way - especially when I crab a delicious dungeness crab from the Pacific Northwest ocean floor.
Someday, you may go doubles. But doubles are more expensive and you have to have more equipment configurations and you will find that most dives can be accomplished with a single al 80, or get steel 100s or even 120s. You can sling an extra 80 if needed. Go doubles if you find yourself really needing them regularly.
Don't do Spare Air. Spare Air is not a redundant air supply. Hopefully, it would get you to the top, but likely you'd just run out of air twice. Like Paddler3D's above post - if you use it in a controlled-non panic situation, it is barely enough. However, in a real situation where you're breathing heavily - it's not enough.
By the way, the Buddy System is the best redundant air supply, and is my first choice. But I've found buddies to be unavailable, lost, or unreliable.
