Cut one cucumber in chunks and put in the blender; whirl with 2 cups of the chicken broth, cilantro, lime juice, salt and Tabasco until smoothly pureed. Peel, pit and coarsely chop avocados. Put into a food processor and add the pureed mixture and the rest of the chicken broth. Whirl very briefly to coarsely puree the avocado, leaving some chunks. Cover and chill at least 2 hours or overnight. Just before serving, finely mince the red onion; seed and chop the second cucumber. Pour soup into 6 bowls and garnish each with a sprig of fresh cilantro. Offer the minced onion and chopped cucumber on the side to add individually to taste.