To make the sauce: Place all ingredients except mayonnaise in a food processor and blend until smooth.Transfer to a small bowl and stir in mayonnaise until well blended. Refrigerate until ready to use.
In a large bowl, combine the mayonnaise, scallions, eggs, mustard,worcestershire, tobasco and salt. Fold in the crab meat, crayfish and 1/2 cup of the breadcrumbs. Form into 2 inch cakes, about 3/4 inch thick. Coat the cakes heavily with the remaining breadcrumbs and transfer to a baking sheet lined with wax paper. Chill, covered for at least 2 hours, or over night.
Heat 2 tbsp of the oil with 1 tbsp butter in a large skillet over medium high heat. Add half of the cakes and cook until crispy and golden, about 5 minutes each side. Add the remaining oil and butter and cook the remaining cakes.