I've seen quite a few people who recommend going down the two (or more) heaters route for the safety value of not having a single heater break. Typically you set the second heater a degree or two lower than the other so it only switches on when really needed.
On the other hand I've read people saying that a bigger heater tends to be more energy efficient. I tend to go this route myself. On my big tank I've got a 1000w titanium heater. I've got a couple spare 300w heaters I could chuck in there if the big one broke - chances are I'd notice a drop in temp before it got to be serious enough to put the fish in danger.
My other concern with the multiple heaters is that while it may save you if one heater dies, it also multiplies the risk of a heater thermostat breaking and a heater not turning off and baking the fish. I've had a couple of heaters die on me over the years - but the only time it killed fish was when a thermostat went and I came home to a bunch of cooked fish in really hot water.
I might not understand your post correctly, so please correct me if so.
The point of having 2 heaters is lost if one of them is actually strong enough to cook the tank. Each is supposed to be small enough so that if it was the only heater, it couldn't possibly cook the fish. It would only be strong enough to maintain the temp at a minimum, for example 80F, even if it was set to 120... Sized correctly, it could stay on 24/7 and never go past 80F, or 85F.
Using this point of view, a single heater that could raise a tank to 100, for example, is already too large.
While it's true that having 2 heaters does in fact double the risk of having an 'exploding heater' the odds are remarkably small for a decent heater. I've kep fish off and on for 30 years and never had an exploding heater. That of course proves little due to it's small sample size, but my guess is that it's quite rare.
By the same token, over heating seems uncommon, but it would likely be much more common that exploding, so the risks are both small, but not the same.
Lasty, it'd be intersting to understand how 1 heater of xxx watts is more efficient than 2 heaters of the same total xxx watts. My understanding is that watts=heat and that is basically the only formula. I don't see how using 2 heaters versus 1 is less efficient in any meaningful way.
