A pony is, of course, totally separate from your main cylinder. Therefore, no matter what goes wrong with your main cylinder, you always have the air in your pony. This is not guaranteed with a manifolded twinset - a single problem can allow you to loose all your gas. So the critics say.
However, the important distinction is that, with a twinset, you can do something about the gas that you’re loosing.
Specifically, if you’re using a twinset, you can reach the valves.
There are, on an isolation manifold, three valves - one for each cylinder, which work just as normal cylinder valves. And also, one on the manifold, which allows you to stop air travelling between the two cylinders. This is the most important one.
So, here’s your scenario: You’re underwater, and suddenly, there’s a *pop* and you’re surrounded by bubbles. You have no idea why, you just know you can’t see, and you’re loosing gas fast.
Immediately, you reach behind your head, locate the isolator, and turn it. Half your gas is now saved. Anything else is a bonus. You now use both hands, one on each of your cylinder valves. You turn both off. The bubbles stop. Carefully, you open the right valve. Nothing happens, you happily continue breathing. You then start to open the left - bubbles galore. Then you notice a hose has burst.
You then (This is the BIG advantage for a twinset) re-open the isolator, allowing air to pass between your cylinders again. You don’t loose any air, because the left cylinder valve is closed. You now have full access to all the air you are carrying. Far from loosing everything, or even half of everything, you’ve lost only a few seconds worth of gas.
Obviously, to gain these advantages, you have to be able to reach all three valves, and to close them reasonably quickly. Keeping the isolator only a single half-turn open is a common method for speeding up a shutdown, as it takes very little time to close the valve, but still allows gas to pass between the cylinders.