You many not realize that the United States government was modelled after the Roman republic, which lasted some 450 years. The branches of the Roman republican government included a senate, assembly (House of Representatives), and consul (President). The executive branch had two consuls instead of a single executive. Consuls where elected simultaneously for one year, and were each given veto power over each other to avoid an accumulation of power.
Our founding fathers debated various aspects of the structures of the branches including whether there should be two presidents. Some felt a single president was essentially a king, while others thought that having two presidents would make each of them too weak to balance a strong legislature. In the end, those worried about the president having too much power were mollified by controls placed on the method of electing the president and his term of office.