try grafting on an additional persons circulatory system to yours. see if your heart can handle the extra load. it can't as it is in equilibrium and its capacity is fine tuned for your body only.
Actually (though I don't know if anyone has ever done this on a human), this works fine in mice. Look up 'parabiosis'.
Totally OT, but there's a lot of research now in this area because they discovered hooking old mice up to young mice causes cellular rejuvenation. A lot of work going on now to figure out what are the exact blood factors responsible for it.
Rejuvenation of aged progenitor cells by exposure to a young systemic environment
http://www.nature.com/nature/journal/v433/n7027/abs/nature03260.html