Bring a large pot of salted water to a boil. Add the scallops and poach gently until they are cooked just to rare, about 2 minutes. Drain well and chill for about 30 minutes, or up to overnight.
Stir together the onions, clam-tomato juice, ketchup, lime juice, cilantro, serrano and hot sauce in a large bowl until well incorporated. Season with salt and pepper.
Quarter the chilled scallops. Add the quartered scallops and shrimp into the onion-sauce mixture and gently stir to combine.
Divide among 4 (6-ounce) glasses. Serve with saltines or tortilla chips.