In a glass or stainless steel bowl, stir together the lime juice, lemon juice, Add the shrimp and scallops, cover, and refrigerate for at least 4 hours and up to 8 hours. Stir once or twice during chilling.
When ready to serve, drain the shrimp and scallops. Add the tomato, cucumber, red onion, garlic, cilantro, Serrano chile, and salt and toss gently.
Just before serving, add the avocado and the juice of one orange and toss. Serve with tortilla chips.
(Note: this recipe can make 8 appetizer size portions or 4 main meal portions)