Questions like these are very difficult to answer, but here's what I think.
The liver has several vital functions in the human body. Due to its complexity and importance, it requires a lot of energy to function. By limiting the body to one liver, the body conserves energy. Furthermore, the liver is already quite large, and has the capability of regenerating itself when damaged. Unlike the kidney, the liver has a repair mechanism, so when it is damaged, it can be repaired. This is not true for the kidneys. Unlike the liver, the kidneys are considerably smaller, and so two optimizes their overall function. But if you notice, the liver also has two lobes.