Melt the butter in a pan over medium heat. Whisk in the flour, and cook to make a smooth paste, about 5 minutes. Gradually stir in the heavy cream. Continue to whisk the mixture until thick and smooth, about 5 minutes. Stir in the shrimp and crabmeat. Add the Parmesan cheese, and stir until melted. Season to taste with salt and white pepper. Serve warm as a dip.