Place lobsters in a large heavy pot. Cover with salted water. Bring to a boil and cook 5 minutes. Reduce heat and simmer 10 to 12 minutes more. Drain and plunge into cold water to arrest further cooking. Extract the meat, chop it and set it aside.
Place the the lobsters' remains in a roasting pan with the onion, celery, carrot, bay leaves and butter. Roast in preheated 400F oven for 45 minutes. Remove from the oven and strain off the butter into a heavy saucepan.
Place the remaining contents of the roasting pan into the 2 quarts water and boil until the liquid is reduced by half. Strain the stock. Heat the butter in the saucepan. Add flour and cook the roux for 4 minutes, stirring constantly. Add the stock and blend well. Then add the cream, Sherry and brandy. Simmer slowly for 30 minutes.
Season with white pepper and salt to taste. Strain the bisque and add the lobster meat. Serve immediately.