arguably the best place for a heart is where it already is, limbs can be compromised to protect your core and unless you have a redundant brain damage to the head would be lethal regardless

@bklassen that doesn’t solve the “needs to be separate location so it doesn’t get damaged at the same time as original heart” requirement.

The heel, obviously...

It goes in the heel. Your warrior can go to war then and get stabbed in the (real) heart and survive. But some guy named Paris then shoots the redundant heart and instead of "two hearts" you live in legends as an "Achiles' heel".

"In the event the first heart goes down, (suppose someone stabs you clean through it, for instance) the second heart kicks in and keeps the blood pumping through your body ..." ... hmmm ... did you mean keeps the blood pumping through your body and gushing out through the first perforated heart ! You might want a quarantine mechanism of the first heart before the second one kicks in!

There's no room in the human torso for a second heart.

@RonJohn We seem to find room for pacemakers and the like. There's also space for visceral fat to be stored - some of us more than others. :) And if you're really short on space, we can survive pretty well without a large chunk of intestines, although the lower abdomen can expand basically as much as it needs to (within the bounds of muscles stretching). Something the size of a house-brick would need a bit of reshuffling, but would be perfectly manageable, albeit with a bulge around your belly. Something the size of a human heart, no problems at all.

@Graham #1Pacemakers are very small and flat. #2 I'm pretty dubious as to how well we'd survive with a lot less intestine. (We already have lot less intestine than our hominid cousins.) #3 Visceral fat is subcutaneous. Very, very exposed, whereas our hearts are protected by a thick, bony shield.

@RonJohn No, visceral fat is, like it says, round your viscera (internal organs). It's very much not subcutaneous. Re lack of intestines, a decent number of soldiers are still around today after being shot in the abdomen and having the damaged bits removed. Oh, and a further argument for how much room there is in the human torso - inhale all the way until you can't get any more air in, then exhale as fully as possible. That's how much space you have to play with. (And that's one reason why visceral fat is bad, because it fills that space up.)

@Graham thanks for the info on visceral fat. Still, you argue against your own points. We (especially soldiers!!) need that space to take deep breaths after physical exertion. If a second heart was where that excess visceral fat is, then people couldn't breathe as deeply and thus couldn't be as physically active. Again, not good for soldiers.

@RonJohn Of course we want as much space for breathing as we can get. If we decide to stick an extra organ in there somewhere, as you say, that gives us a trade-off for whether the improved survivability is worth the hit on aerobic performance. It's a trade-off that's already made for soldiers though, if you think about the weight of body armour and other kit they're carrying - they've already decided to trade athletic performance against survival. This would be just another similar decision (albeit with more surgery!).

@Graham only certain soldiers in certain specific situations have made that trade-off. If the magical new heart just sprouts up as a replacement, why not have the damaged heart simultaneously shrink? Heck, KISS and have the magic repair the existing heart!!

@RonJohn It's magic, so basically anything can happen. :) The OP has given us some ground rules for what his magic can do, so we kind of want to stick within them (or suggest why his magic system is internally inconsistent as a frame challenge, of course). That's the problem with magic - having it not do everything just because it's magic. (To quote Pratchett misquoting Heinlein, there ain't no such thing as a free goblin.)

In F. Paul Wilson's debut novel 'Healer' there's something like this. A redundant small heart is added to the main character that's located in his pelvis. When he is shot in the primary heart it keeps him alive until he can recover.

Pak Protectors have 2nd heart in groin afair. Note supply of Thalium Oxide needed to ensure tree of life viability. :-).

In one of the Salvatore's Dark Elf books, the crazy wizard faced Mind Flayers, and those are known to psi-blast their opponent's head, causing damage to the brain. In preparation to this, the wizard wisely moved his brain from his head to his ass.

Reading some of the answers, you might like to ask about heart armour (that is, under-your-rib-cage casing for the heart itself

Or you could just not get stabbed in the heart in the first place. Maybe put something really hard around it to protect it.

@RussellMcMahon Came to this question expecting Ringworld reference, not disappointed.

@IanKemp - :-). - I failed to note that a Pierson's Puppeteer has it's brain in neither of its heads but in an area under an armoured hump in the upper front of its body. The creators fails to note (AFAIR) how the high bandwidth eye-brain optical path is dealt with. [ I've just finished a fast rerereread of "The Ringworld Engineers".

@RonJohn compare human heart size to fully grown fetus?

@msouth women carrying fully (or even partially) grown fetuses are known more for weight gain, sore backs and cravings for pickles instead of dexterity and martial prowess.

@RonJohn Not when it's the size of a human heart, and presumably no pregnancy hormones would be included.

It isn't only the heart stopping that kills people, it's the bleeding. You will bleed many liters of blood from a sword stab in the heart and your second heart won't solve it. It will only give another place to stab you and make you lose liters of blood.