It has never been done. The chance of someone to be the top in the world at 2 separate (very different) sports is astronomical.
I won't be surprised if it happens sometime soon though :)
Japan first participated in the Summer Olympics in 1912. They have participated in all Summer Olympic since except for 1948 and 1980. They first participated in the Winter Olympics in 1928 and have been in all Winter Games except the 1948 Games. The 2008 Olympics is the 20th Summer Games Japan has participated in. They have been to 18 Winter Games.
South Africa has participated in 19 summer Olympic games and 6 winter Olympic games.
Jamaica has participated in every Summer Olympics since 1948 and participated in the Winter Olympics from 1988-2002.
80 countries and 2 territories participated in the 2010 Winter Olympic Games in Vancouver, Canada. More countries compete in the Summer Olympics than Winter.
England along with Switzerland and France are the only three countries to have participated in every summer and winter Olympic games. English athletes have won 780 medals in summer and 22 medals in winter Olympics games.
Through the 2010 Winter Games in Vancouver, Jamaica has participated in 21 Olympics ... 15 Summer Games and 6 Winter Games.
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82
Summer
Yes, Austria has participated in all 21 Winter Olympics (through the 2010 Games in Vancouver).
Belgium has participated in both Olympics i got my answer from wikipedia
Through the 2008 Games in Beijing, Kenya has competed in 15 Olympic Games, 12 Summer Games and 3 Winter Games.