Pat the shrimp and squid very dry with paper towels. Dry seafood fries; wet seafood steams the pancake.
Whisk the flours, salt, cold water and 1 egg until smooth and thin, like heavy cream. Whisk the other 2 eggs in a separate bowl.
Stir together the soy sauce, rice vinegar, gochugaru and sesame seeds for the dipping sauce.
Heat 2 tbsp oil in a 26 to 28 cm nonstick or cast iron skillet over medium-high heat. Spread the scallions in one even layer, then pour the batter over them.
Scatter the shrimp and squid on top and press them lightly into the batter. Cook 3 to 4 minutes without moving, until the edges are crisp.
Pour the 2 beaten eggs evenly over the top. Cook 1 more minute, then slide a spatula under and flip in one motion.
Drizzle the remaining 1 tbsp oil around the edge and press the pancake down. Cook 3 to 4 minutes until golden and the seafood is cooked through.
Slide onto a board, cut into squares with scissors and serve hot with the dipping sauce.