Magic lets you take a lot of shortcuts when it comes to fighting, and occasionally do things that normal people may refer to as 'cheating', such as installing a second, redundant heart. 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.
The redundant heart itself is somewhat magical in nature to let it actually work a bit better. It's a dormant organ, that upon the demise of the first heart, sprout and injects itself into the vein and artery network of a human being. It can also, regardless of position, supply blood to and from the lungs. Also, being magical, it has no problem doing this and is capable of matching the original heart's flow regardless of where it is on the body, because magic. (The downside is that this redundant heart only works for 24 hours, at which point the original heart must take back over.)
Now, the question is: where should this redundant heart be placed? It must take up room, after all. A bit of organ rearrangement is fine, and the heart can be placed anywhere there's tissue. You can even grow a place for it, for instance, if you want to have it placed in your hand, you can have it grown as a bulge in the palm of the hand. Presumably you'd just use that arm as a shield arm. (Which isn't a bad idea, but since that hinders the use of the hand outside combat, it's not really what I'm looking for.)
The goal is to place the heart somewhere that, during the course of a normal battle, won't be stabbed by accident, but also won't inhibit normal motions.