If you want to speed up the marination process, you can blanch the shrimp and scallops for a couple of minutes until barely cooked. Drain and chop shrimp and scallops. If you prefer working with raw seafood, chop shrimp and scallops without blanching.
In a large mixing bowl, combine shrimp, scallops, red onion, jalapeno, tomatoes, cilantro, mango, pineapple, lime juice, salt and pepper. Mix well.
Refrigerate the ceviche mixture for at least 4 hours to let the flavors incorporate. For better flavor, let it sit overnight.
Just before serving, add the chopped avocado. Serve with tortilla chips.