Melt 3 tablespoons of the butter in a 6 quart Dutch oven or large soup pot over medium high heat. Add shrimp and scallops and cook until all the shrimp pieces have just turned pink and are opaque, stirring frequently (about 4 minutes).
Remove shrimp and scallops with a slotted spoon and place in a large bowl, leaving juices in pot.
Add the remaining 3 tablespoons of butter to the pot with the juices and melt. Add celery and cook for 4 to 5 minutes, or until tender (stirring frequently). Add garlic and cook until fragrant, about 30 seconds, while continuing to stir.
Slowly add flour while whisking to combine. Pour in half-and-half and seafood stock, while continuing to whisk.
Bring to a simmer, stirring frequently. Simmer for 10 minutes, or until thickened, stirring frequently. Reduce heat as necessary to maintain a simmer (medium to medium-low).
Return shrimp and scallops to pot. Add wine, Old Bay Seasoning, nutmeg, sugar, salt and pepper. Stir well to combine.
Simmer for an additional 5 minutes, stirring occasionally.
Sprinkle with chopped fresh parsley when serving, if desired.