Wash the seafood and pat dry, then place in a lidded container.
Place the olive oil, garlic, lemon zest, orange zest and parsley in a small bowl and stir to combine. Pour the marinade over the seafood, massaging it into the shrimp and scallops with your fingertips. Cover and marinate in the fridge for at least 1 hour, but no more than 2 to 3 hours.
If using bamboo skewers, soak them in water for 15 minutes before using, so they don’t scorch during cooking.
About 15 minutes before cooking, remove the seafood from the fridge to bring it to room temperature. Thread the shrimp and scallops onto separate skewers (as they have different cooking times), being careful not to crowd them on the skewers.
Heat a barbecue or chargrill pan to medium-high. Cook the shrimp skewers first, by brushing one side with olive oil and placing that side down on the hotplate or pan. After about 2 minutes, carefully flip them over using tongs, then cook for another few minutes until they turn a nicely striped orange. Transfer to a warmed plate.
Cook the scallop skewers in the same way, but reduce the cooking time to about 1 minute on each side. You don’t want to overcook scallops as they can turn rubbery. Serve warm, with lemon wedges and scattered with extra parsley.