Melt butter over medium heat in a large stock pot; add apples and onion. Sauté, stirring occasionally for 8 to 10 minutes. Add chicken broth; bring to a boil, cover and simmer for 15 minutes. Place mixture in blender; purée until smooth. Return mixture to the pot; stir in Cheddar cheese, Brie cheese, apple juice concentrate and heavy cream. Heat until cheeses melt and soup is smooth and hot. Season with salt and pepper.