Pat the fish and shrimp very dry and put them in the freezer for 15 minutes while you make the spice paste. Cold seafood blends into a proper sticky paste. Warm seafood turns to mush.
Blend the shallots, garlic, chilies, candlenuts, galangal, turmeric, coriander, white pepper and shrimp paste with 1 tbsp of water to a smooth paste. Heat 1 tbsp oil in a small pan over medium heat and fry the paste for 4 to 5 minutes, stirring, until it darkens, smells cooked rather than raw, and the oil separates at the edges. Scrape into a bowl and cool completely.
Toast the coconut in a dry pan over medium low heat, stirring constantly, for 3 to 4 minutes until light gold. Tip onto a plate to stop it cooking.
Put the cold fish and shrimp in a food processor and pulse in short bursts, 10 to 15 times, scraping down once, until you have a coarse, sticky paste with no large chunks. Do not run the motor continuously.
Add the cooled spice paste, toasted coconut, kaffir lime leaf, sugar, salt and lime juice. Pulse 5 or 6 more times until just combined. The mixture should be tacky and hold together when you squeeze a spoonful.
Fry a teaspoon of the mixture in a small pan and taste it. Adjust salt or lime now, because you cannot fix it after the skewers are shaped.
Wet your hands. Take about 1/4 cup (75g) of mixture, flatten it in your palm, lay a lemongrass stalk or skewer on it and squeeze the paste around it into an oval about 4 inches long and 1.25 inches thick, leaving a handle at one end. Repeat to make 12. Chill the shaped skewers for 15 minutes if you have time; they hold their shape better.
Grill: heat a grill or grill pan to medium high and oil the grates well. Brush the skewers with oil and grill for 10 to 12 minutes, turning every 3 minutes, until deeply golden with charred spots and firm to the touch. Broiler: place on an oiled rack over a tray, 4 inches from a high broiler, 5 to 6 minutes per side. Air fryer: 390F / 200C for 10 to 12 minutes, turning halfway, in a single layer.
Rest for 2 minutes and serve with lime wedges and, if you have it, sambal matah (thinly sliced shallots, chili, lemongrass and lime with a spoon of hot oil poured over).