That's down to the physical fact that one heart pumps blood to the lungs to get oxygenated and the other to push that oxygenated blood around the body. That enables us to support a larger, more active body, and incidentally allows us to develop larger brains because brains need a prodigious amount of oxygen. So without two hearts we wouldn't have a two-phase circulation system and we wouldn't have been able to grow a big brain and do exciting stuff like global warming. Birds on the other hand, got quite smart with small brains (because once you start flying around you don't have spare oxygen for a big brain, so there's not always a single evolutionary route). People always talk about evolution like it's purposeful, of course, and it isn't. Things happen because they can, and if they're useful in a given circumstance, they stick around.
The combined heart comes about through a bit of developmental origami, the initial heart begins as two identical tubular structures in the embryonic mesoderm that come together and are twisted into a knot (ok, more of a tight fold around one another).
Liver is actually not really two livers, it more complicated than that. I don't remember. It involves budding and features a guest appearance by the gall bladder. Pancreases are weird too, but you knew that. Basically specific cells in an embryo scoot around, following signalling gradients created by the carefully orchestrated expression of genes and transcription factors, seasoned with a dash of feedback. It's amazing and the essential parts of these processes evolved hundreds of millions of years ago, everything thereafter is tinkering.
There are people who have combined kidneys, that have wrapped around one another akin to the heart. If you're a pregnant woman, you're carrying the ova that will be your grandchildren. Primordial germ cells, as they like to be known, go on quite a trek around the embryo, so we're all basically cellular hobos.