Combine first 4 ingredients in a saucepan; cook over medium-high heat 30 minutes or until thickened and reduced to 2 cups; set aside. Cook manicotti according to package directions; drain and set aside. Heat butter in a large Dutch oven over medium-high heat; add onion and next 5 ingredients; cook, stirring constantly, 5 minutes or until tender. Add shrimp and crabmeat, and cook, stirring constantly, 5 minutes or until shrimp turn pink. Cool 10 minutes, and drain well. Combine seafood mixture and whipping cream mixture. Fill manicotti shells, and place in 2 lightly greased 11x7 inch baking dishes. Sprinkle with cheeses, and cover with foil. Bake at 350 degrees for 15 minutes. Uncover and bake 10 additional minutes. Serve immediately.
Ready in 2 hours - 1 hour prep