Juice the lemons into a bowl, extracting as much juice as possible.
Dice the halibut and shrimp into bite-sized pieces.
Mince the garlic cloves and finely chop the onion.
Dice the cucumber and chop the hot pepper finely.
Chop the tomatoes and measure the chopped parsley and cilantro.
Place the halibut, shrimp, garlic, onion, cucumber, hot pepper, parsley, cilantro, and tomatoes into a non-reactive bowl.
Pour the lemon juice over the mixture, making sure the seafood is fully submerged.
Stir well to coat all ingredients evenly.
Cover the bowl and refrigerate for 8 hours or overnight until the seafood becomes opaque and firm.