This is what we found on Wikipedia regarding the South Summit:

The South Summit is a subsidiary peak of Mount Everest in the Himalayas between the South Col (at 7,906 metres (25,938 ft)) and the main summit (at 8,849 metres or 29,032 feet) above sea level. Although the South Summit's elevation of 8,749 metres (28,704 ft) is higher than the second-highest mountain on Earth (K2 at 8,611 m or 28,251 ft), it is not considered a separate mountain as its topographic prominence is only 11 meters.