Combine sugar and water over medium heat. Stir until combined and remove from heat. Let cool for 10 minutes.
Stir in lemon juice and lime juice until well combined.
If using right away, stir in 1-2 ice cubes to reduce the temperature. Otherwise decant into a jar or bottle and store in the fridge for up to 2 weeks.
If a frothier sour mix is desired (such as for whiskey sours), you can mix in 2 egg whites to the cooled mixture. Shake well to combine. If adding egg whites, use within 2 days.
Notes
Use fresh lemon and lime juice for the freshest taste, but the bottled concentrate will work perfectly fine. For fresh lemon juice, you will need about 6 lemons. For fresh lime juice, you will need about 8 limes. Feel free to mix and match with the ratio and/or try other citrus juices. This recipe is easily scaleable up or down. Use ½ cup measures for a smaller batch, or double it if you are planning a party.