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'. Conjoined (that's conjoined, not coinjoined
) twins are identical twins[1] joined in utero. A rare phenomenon, the occurrence is estimated to range from 1 in 49,000 births to 1 in 189,000 births, with a somewhat higher incidence in Southwest Asia, Africa and Brazil.[2] Approximately half are stillborn, and a smaller fraction of pairs born alive have abnormalities incompatible with life. Thoraco-omphalopagus (28% of cases):[6] Two bodies fused from the upper chest to the lower chest. These twins usually share a heart, and may also share the liver or part of the digestive system.[7]
https://en.m.wikipedia.org/wiki/Conjoined_twins