Combine the granulated sugar and 1 cup water in a small saucepan over medium heat. Stir until the sugar fully dissolves and the mixture just comes to a simmer (do not boil hard).

Once the sugar is dissolved and the syrup is hot, add the lemon zest and lime zest. Reduce heat to low and steep the zests in the syrup for 2–3 minutes to extract oils, stirring occasionally.

Remove the pan from heat. Stir in the fresh lemon juice, fresh lime juice, and citric acid (if using). Taste and adjust: add up to another 1 tsp citric acid if you want more tang, or 1–2 tsp water if too intense.

Let the syrup cool to room temperature (about 20–30 minutes). Strain the syrup through a fine mesh sieve to remove the zests and any solids. Transfer the strained syrup to a clean container and refrigerate until cold (about 30 minutes). The syrup will keep in the fridge for up to 1 week.

Assemble Drink

Fill four serving glasses with about 1/2 cup ice cubes each (or adjust per glass).

To each glass add 2 tablespoons of the chilled citrus syrup. (For a milder flavor use 1 tbsp; for stronger, use up to 3 tbsp.)

Pour 1 cup of chilled club soda (or carbonated water) into each glass over the syrup and ice. Stir gently once to combine.

Garnish each glass with a lemon slice and a lime slice. Serve immediately. Stir again before drinking if syrup settled.