Combine the light brown sugar and water in a small saucepan and heat gently over low heat, stirring just until the sugar fully dissolves; do not boil. Remove from heat, add the orange and grapefruit zest, cover, and let steep for about 10 minutes to extract citrus oils without bitterness, then strain through a fine-mesh sieve and allow the syrup to cool completely.
Pour the cooled citrus syrup into a large pitcher, add the fresh orange juice, fresh grapefruit juice, and the fine sea salt, then stir until fully combined. Pour in the ice-cold water and mix gently until the drink is uniform and clean.
Refrigerate for 1–2 hours until fully cold and balanced, stirring once before serving.
Fill glasses with ice, pour over the grapefruit orange drink, and garnish with orange slices and grapefruit slices.
