GDI
I'm a firm believer in redundancy, especially if you're planning on taking this to the level that requires multiple stages. You're going to get a lot of opinions on this subject due to the many different ways a diver can configure his gear. Some configurations may even have you scratching your head and wondering how or why would someone dive this way. But what it really boils down too is what configuration works for you.
If you can borrow someone's rig and stages. And don't get little 30's or 50, but steal stages in the 100 cu ranges. Go to a shallow lake or body of water some were, with a buddy, dressed and in the water with you, and try this little test. Wear what you would normally wear on a cold-water dive with twins and these large stages, full not empty. Dump all the air in the BC and your suit and now try and stay on the surface with your hands on your head for two minutes. Now, take this same rig to depth, 100ft. or so and dump all the air in the BC. See if you can fin up to the surface with all this gear on. Now, remember, your simulating a deep dive, where you've just had a BC failure and you've got deco obligations, your ceiling is at 70ft and your going to do your first switch to deco gas at a 100 because what your carrying allow for this.
So, you're going to deploy a lift bag to help you get up. OK, you better be able to control the gas expansion in that bag and keep your hands free for the gas switch or you may miss your ceiling. Or, your going to use your drysuit, so you've got to put enough gas in to allow you to fin up and risk the neck seal burping and loosing your lift. Which would start you descending again causing you to use more of the gas in the argon bottle, which may only be a 13cu tank. I personally dive with a 6cu tank 'cause I'm just a little guy. Along with descending, now your getting ear squeeze, mask squeeze, your mind is starting to play games (task loading is increasing at a rapid rate). Right about now you start wishing you had that double bladder BC.
My instructor had a saying. At depth, two is one and one is none. I use this when I configure my rig and assess the risk of eliminating equipment for the sake of cost or pear pressure. Like I said, you're going to get a lot of opinions on this subject. Go in with your eyes wide open and don't be afraid to change or question ones reasons for doing something.
Silverback
