This shellfish ceviche amps up the flavor by substituting cucumber juice for some of the citrus. For best results, use a small, think-skinned cucumber variety, such as Korean or Delikatesse. Read more about cucumbers on Wall Street Journal's "Off Duty" section.
Cut half of the cucumbers into ¼-inch dice and set aside. Peel and seed remaining cucumbers and place in a blender along with lime juice, orange juice, and a pinch of salt. Blend for 1 minute, then pass mixture through a fine mesh sieve and discard the pulp.
Bring a saucepan of lightly salted water to a simmer and poach the shrimp and scallops in batches until barely cooked through, about 1 minute. Drain in a colander and spread on a baking sheet to cool.
In a large, nonreactive bowl, toss the cooled shrimp and scallops with the tomatoes, chiles, cilantro, and onion. Add reserved juice and toss again. Cover and chill for at least 2 hours.
Season ceviche to taste with salt and additional lime juice, if necessary. Spoon into small glasses, garnish with cilantro, and serve.