Bring a saucepan of salted water to a boil over medium heat. Add shrimp and cook until they just turn pink, about 1 1/2 minutes, then transfer to an ice bath. When cool enough to handle, chop roughly and put in a large plastic bag with scallops. In a small bowl, whisk lime juice, lemon juice, sugar and salt together until the salt is dissolved. Stir in jalapenos, cilantro, and red onion to combine, then add the mixture to the bag with the scallops and shrimp. Put the bag in the refrigerator to marinate for at least 2 hours before serving.