[Mark Davies]

<font color='#000080'>I think you're going to get as many opinions as there are divers!

I've been considering this issue myself recently. After speaking with twins divers I am thoroughly convinced by their merits. You need to decide whether you want an entirely separate air source for bail out or just require redundancy for system failure. Of course, the belt and braces approach is the one suggested above - twins and a pony!

In my opinion, deciding you need a bail out air source is planning for bad gas management. Agreed, there may be situations were you are sufficiently distracted by ongoing events to not notice that you are running out of air. Suddenly sucking on nothing would be one hell of a wake-up call and you'd be glad of your pony then!

Can you anticipate being in this situation? The answer to that should tell you whether you need a pony. A twin set alone would not cover you in this situation. That's why twin divers use the rule of thirds (i.e. plan to surface when tanks are still 1/3 full). Would twin 7's then be enough air for you, or will you need 10's?

It's a can of worms, ain't it? I know I've not been much help, but there seems to be no absolute answer. I hope that I have at least given you something to think about.