Complex sentence: Calhoun thus became the second of two vice presidents to serve under two different presidents, the other being George Clinton, who served as Vice President from 1805 to 1812 under Thomas Jefferson and James Madison.
Split into simpler sentences: Calhoun thus became the second of two vice presidents to serve under two different presidents. The only other man who accomplished this feat was George Clinton, who served as Vice President from 1805 to 1812 under Thomas Jefferson and James Madison.