Preheat oven to 400°. Unfold 1 pastry sheet and place on a parchment-lined baking sheet; repeat with remaining pastry sheet. Whisk egg and water; brush over pastries. Using a sharp knife, score a 1-in. border around edges of pastry sheets (do not cut through). With a fork, prick center of pastries. Bake until golden brown, about 15 minutes. With a spatula, press down center portion of pastries, leaving outer edges intact. Remove to wire racks to cool., Meanwhile, for topping, arrange rhubarb in a single layer in a 13x9-in. baking dish. Combine orange juice, honey and amaretto; pour over rhubarb. Bake at 400° until rhubarb is just tender but still holds its shape, about 10 minutes. Remove with a slotted spoon, reserving cooking liquid; let rhubarb cool. Transfer reserved cooking liquid to a small saucepan; bring to a boil over medium-high heat. Reduce heat; simmer until reduced to 1/2 cup, about 20 minutes. Cool., For filling, stir together mascarpone cheese, amaretto and honey until smooth. Spread mascarpone mixture over center of each pastry. Top with rhubarb ribs. Brush rhubarb with cooled cooking liquid. Refrigerate leftovers.