Melt 2 tablespoons butter in a saucepan over medium heat. Cook and stir shallots until translucent, about 5 minutes. Pour in white wine; simmer until reduced by almost half. Strain out shallots; discard.
Pour milk, heavy cream, and tarragon into the saucepan; simmer until slightly thickened, about 5 minutes. Season with salt. Reduce heat to low and cover to keep warm.
Melt 2 tablespoons butter in a skillet over medium heat. Cook and stir fennel until slightly softened, about 5 minutes. Pour in orange juice; simmer until slightly reduced, about 3 minutes. Stir in spinach. Reduce heat, cover skillet, and cook spinach until softened, about 5 minutes. Stir well. Remove from heat and keep covered.
Season shrimp and scallops with salt and pepper.
Melt 2 1/2 tablespoons of butter in a cast-iron skillet over medium heat. Add scallops; cook until browned, about 2 minutes. Flip and cook on the second side for 1 minute. Add remaining butter and shrimp to the skillet; cook until shrimp turns pink, 3 to 4 minutes.
Spoon some of the cream sauce onto each serving dish. Divide fennel and spinach mixture evenly among dishes. Arrange 3 scallops and 3 shrimp in a spiral over the fennel and spinach mixture.