Place the sugar and water in a small saucepan and heat over medium heat, whisking constantly until the mixture becomes clear, about 1 minute. Remove from the heat. Cool to room temperature (takes a few minutes).
Stir in the lemon juice and lime juice, then pour into a jar. Keeps 1 week in the refrigerator (the flavor is best in first 24 hours, but it’s still good over a week).