Melt half the butter in a large saucepan over medium heat. Add the onion and cook, stirring frequently, until soft, about 5 minutes. Sprinkle the butter and onion with the flour and quickly stir it to combine. Cook, stirring constantly, for 2 minutes. While stirring, slowly add the clam juice and whipping cream. Mix well. Add the salt, white pepper, and cayenne if using. Cook, stirring constantly, for 3 minutes or until the sauce has thickened. Reduce the heat to low. In a skillet, melt the remaining butter over medium heat. Add the shrimp and cook for 2 minutes, stirring frequently. Add the crab meat and cook for 1-2 minutes or until the shrimp is cooked. Add the shrimp, crab, and butter to the cream sauce. Mix well. Serve the Alexander sauce immediately.