Keep clams cool until ready to cook. Place shrimp in bowl. Mix lemongrass, ginger, garlic, sugar and pepper. Add half to shrimp along with 1 teaspoon fish sauce and toss well. Refrigerate until ready to cook, up to 8 hours. To remaining lemongrass mixture, add scallion whites, remaining fish sauce, soy sauce, wine and water.
Preheat oven or grill to 450°F. For oven, place large sheet pan on middle rack. Using 2 sheets of foil on top of each other, lift the shorter sides and press together lightly to form a loose packet with wings, open at the top. Do not press tightly yet. Make a 2nd packet in the same manner. Divide clams between the packets. Pour half the lemongrass mixture over the clams in each packet. Distribute into a single layer. Spread shrimp over the top of clams. For each packet, bring together long edges of foil over the top of clams and seal tightly. Open the pressed edges and seal tightly with the top edge. Place both packets onto hot sheet pan in oven, or directly on grill rack. Cook 20-25 minutes in oven or 15-20 minutes on grill, until clams are fully open.
While clams roast, cook pancetta in medium skillet until crisp. Open bags carefully and transfer clams and shrimp to serving bowls, along with the cooking juices. Top with crispy pancetta and sliced scallions. Serve warm with rustic bread for dipping in sauce.