In a bowl, whisk together the citrus juices, cumin, mint, salt, pepper, cayenne, sugar and oil. Add the shrimp, crab, green onions and celery and gentle mix to combine. Marinate mixture in the fridge 30 minutes. Arrange salad greens on 4 plates. Arrange orange slices in a ring around the outer edge of the greens. Spoon the seafood mixture into the centre of the plate. Garnish with mint and serve