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Your preferences may evolve. I started with a single 72 when buddy breathing was taught. I later added an octopus. Then I added a pony bottle. Next came twin 50's. Then a manifold to use 2 regulators. And finally I changed to valves-down. My 72 is too long to comfortably wear valve-down.
sometimes i browse ebay just to see if there are any deals, and a couple of times i've seen 7l twin sets. i just think it's weird that you wouldn't just use a single 15l tank. is there any advantages?