Make the batter: In a large bowl, whisk together the rice flour, tapioca starch, and salt. Make a well in the center, pour in the coconut milk and 2 cups water, and whisk until the batter is smooth. Whisk in the scallions; set batter aside to let rest for 1 hour.
Make the filling: Drain the mung beans. Bring 1" water to a boil in a pot fitted with a steamer attachment. Add mung beans to the steamer, cover, and steam until just tender, about 10 minutes. Transfer mung beans to a plate and set aside to let cool.
Heat 2 tbsp. oil in a 12" nonstick skillet over high heat. Add shrimp, pork, and onions and cook, stirring occasionally, until pork is golden brown and cooked through, about 3 minutes. Season with salt and set shrimp mixture aside.
Working in batches, heat 2 tbsp. oil in a 12" nonstick skillet over high heat. Using a liquid measuring cup, pour 1⁄2 cup batter into the skillet, swirling to cover the bottom. Cook until partially set, about 45 seconds. Sprinkle 2 tbsp. mung beans over crepe; then add one-sixth of the shrimp mixture and 2⁄3 cup of the bean sprouts. Lower the heat to medium, cover, and cook until the bean sprouts have wilted slightly, about 3 minutes. Uncover and drizzle 1 1⁄2 tsp. oil around the rim of the skillet. Cook, uncovered, until the edge of crepe crisps and pulls away from the rim of the skillet and turns golden brown, about 3 more minutes. Fold crepe in half with a spatula and slide onto a platter. Wipe out skillet and repeat to make 6 crepes in all.
To serve, pass the platter of crepes with the garnishes and dipping sauce. Tear off a piece of the crepe, wrap it with lettuce and any combination of the herbs, and dip in nuoc cham, if you like.