PS It normally a small spare tank, say an AL19 or so, rigged either to you main tank (normally a single AL80 tank) with another regulator on it, or, you can rig it on your side of your harness or BC in the same style as a stage or deco bottle.
Its 'redundant' because now you have TWO first stages on separate tanks and TWO second stages as well. And two separate K-valves as well. Were one valve to ever fail (pretty remote odds) or if one of the first or second stages were to fail, you'd have a spare.
Note: A PONY does NOT mean its EXTRA air, in that if you use 2500 psi of your 3000 psi air supply, you say "Oh I want to dive longer, so I'll use my PONY for stay down some extra time." A PONY is 'dedicated' air ONLY for an Emergency Situation - to surface with. It IS NOT used to EXTEND Bottom Time! Proper use of the SPG is imperative and using a PONY is not an acceptable excuse to run out of air!
On the 'CON' side of the coin, it adds an extra expense of having ANOTHER tank, another regulator, another annual inspection, another SPG. Also, it should be where you can see the SPG and verify you DO, in fact, have air when you NEED it. Mounted on your single AL80 on your back, it could trickle with a slow leak and you would never know it until you went to use it and - Walla, NO AIR! It can also add to confusion during an OOAE in as far as "Oh boy, ok, which regulator do I grab now?"
You have to weight the pro's and con's for yourself.