Preheat oven to 500°F Melt butter in heavy large skillet over medium-high heat. Add mushrooms and shallots; sauté 5 minutes. Add cream, wine and broth. Boil until mixture is reduced to 1 cup, about 12 minutes. Mix in mustard and vinegar, then spinach. Season with salt and pepper. Cool.
Roll out 1 pastry sheet on lightly floured surface to 12-inch square. Place 1/8 of spinach mixture on bottom half of 1 square. Top spinach mixture with 2 shrimp and.
2 scallop halves. Brush pastry edges with water. Fold unfilled half of pastry over filling, forming rectangle. Press edges of pastry closed with tines of fork. Place pastry on large baking sheet. Repeat with remaining pastry, spinach mixture, shrimp and scallops.
Bake pastries 5 minutes. Reduce heat to 375°F Bake pastries until puffed and golden brown, about 15 minutes longer. Serve warm.