There are no stupid questions.
The way a set of manifolded doubles works is that either regulator has access to all the gas in both tanks, so long as all the valves are open. Closing either of the outside valves stops the gas flow to that regulator -- so, if you have a freeflow or a big leak, you can close down that valve, and the other regulator still has access to all the gas in both tanks. If you have a leak from the valve itself, or the manifold, you can close the knob in the center and isolate the tanks from one another. That way, you only lose half your gas, because the tank away from the leak is isolated from it. (That's why the center valve is called the "isolator".)
That's the primary value of diving doubles -- redundancy. When you are doing very deep dives, or diving under a rock overhead, you can't afford to have a single regulator failure force you to surface, because regulator failures, although not an everyday occurrence, aren't actually rare. Manifolded doubles are one way of providing redundancy to the diver, but you'll notice that only works if you can actually reach back and close those valves when you need to, and do it quickly. It takes some practice to learn to do that, especially learning to do it without changing depth or losing awareness of your buddies and where you are.
If high gas consumption is your reason for thinking about double tanks, you're generally better off working on the things that are causing the high consumption, or switching to a larger single tank. Doubles have a bunch of disadvantages -- they are heavy and hard to move around on land, often cost more to fill (many shops charge you for two fills, even if you're only putting one tank's worth of gas in the setup), and often don't fit well on charter boats. The primarily usage of double tanks is for people who are diving deep or diving in caves, and they often use dry suits because deep water is cold, and because they like the redundant buoyancy of having a wing and a dry suit, in case one isn't working properly.