In a bowl, mix crab, shrimp, cucumber, tomato, chili sauce, Dijon mustard, mayonnaise, lemon juice and pepper. Cover and refrigerate for 1 hour.
Slice the avocados lengthwise and remove the pits. Scoop out the avocado, leaving some around the edges. Dice the meat of one avocado and add the seafood mixture along with the chopped cilantro. Slice the meat from the second avocado and use to garnish plates.
Spoon the mixture into the avocado halves and serve.