Heat 1 tablespoon oil in a 12-inch nonstick skillet over moderately high heat until hot but not smoking, then sauté chili and white parts of scallions, stirring occasionally, until lightly browned.
Transfer with a slotted spoon to paper towels to drain.
Pat scallops and shrimp dry separately and season with salt.
Heat remaining 1 1/2 tablespoons oil in same skillet over moderately high heat until hot but not smoking, then cook scallops until browned, 2 to 3 minutes on each side (scallops will be almost cooked through).
Transfer scallops to a bowl with slotted spoon and sauté shrimp in same skillet, stirring occasionally, until almost cooked through, about 3 minutes.
Add shrimp to scallops.
Add coconut milk, curry paste, broth, brown sugar, fish sauce, and lime juice to skillet, then simmer, stirring occasionally, 5 minutes.
Meanwhile, cook linguine in a 6- to 8-quart pot of boiling salted water until al dente, then drain in a colander.
Stir scallops and shrimp with any liquid in bowl into sauce in skillet and heat to boiling.
Reduce heat and simmer until scallops and shrimp are just cooked through, about 2 minutes.
Transfer seafood to a clean bowl with slotted spoon and add linguine and cilantro to sauce in skillet, tossing to coat.
Divide pasta and sauce among 4 bowls.
Top with seafood and sprinkle with scallion greens and chili mixture.