Yooper, buoyant tanks or neutrally buoyant steel tanks were in vogue 40 odd years ago because it was thought that such tanks, when empty, would assist the diver back to the surface. There were no BCD's. NOW, that BCD's are available, I would have thought that the buoyant tank would go the way of the dino and BCD's would allow more flexibility. Yet, even with this modern answer to the buoyancy control problem, we're hearing "don't trust it". There is such a thing as being too cautious.
BTW, Walter is right. Twin 70's are not particularly heavy. They are neutral to slightly negative. Note that they were in use everyday BEFORE there were ANY BCD's at all. My bud has a set of these dated 1954. I used twin 70's with a wet suit and often with nothing but swim trunks, mask and fins.
There is no single rule about tanks, including steel tanks of which there are a great variety. As far as our English poster's question, be aware that twin tanks will SLOW you down, be prepared to increase air consumption if you do any swimming. Perching on a wreck is the best place for twins. Make those low pressure steel tanks, please. The HP Fabers will sink you like a rock, thereby vindicating Yooper. Twin aluminum will float like a cork making you want to strangle Yooper.
Seriously, I suggest twin 12L Faber. Fill them to 200 bar. This setup should provide enough air for two non-decompression dives to normal depths. It will also allow a single deco dive with a short stop, 15 minutes or so with reserve air. If you are a weight lifter or Iron Man competitor, and fairly tall, you might consider a slightly larger set. I am assumming you will encounter swells and various weather conditions on your trips. Moving around wet decks and climbing ladders can be hazardous if you are overloaded. Use common sense.
There is no time scale for purchase of a set of twin tanks. However, you should have 20 or so OW dives logged before doing deco dives.
I don't know the answer to your BCD question. A dive shop pro should know whether a kit is available. If anyone advises you to use a "backplate", make sure you figure its weight in your buoyancy estimate.
. Ultimately, what you're after is a balanced rig, which basically means that you should be able to swim up your full tanks with no aid from your BC. It ain't gonna happen with double steel tanks and a wetsuit.
