In a 5- to 6-quart pan over high heat, bring 4 quarts water to a boil.
Rinse crabs and fish and pat dry; cut fish into 1- to 1 1/2-inch pieces.
Add butter and olive oil to a 12-inch frying pan (with sides at least 2 1/2 in. tall) or 14-inch wok over medium-high heat; when butter is melted, add garlic and stir just until fragrant, 1 to 2 minutes. Add fish and turn pieces occasionally until beginning to brown, 2 to 3 minutes.
Pour in sherry and lemon juice; gently add crabs. Sprinkle with parsley. Cover and simmer until crabs are hot and fish is opaque but still moist-looking in the center (cut to test), 5 to 6 minutes. Add salt and pepper to taste.
Meanwhile, add angel hair pasta to boiling water; cook, stirring occasionally, until barely tender to bite, 3 to 4 minutes. Drain pasta well and spread in the bottom of a wide serving bowl.
Pour crab mixture over pasta and garnish with lemon wedges. Serve with baguette slices to sop up the sauce.