Combine 1 cup sugar and 1 cup water in a small saucepan and bring to a boil over medium heat, stirring until the sugar dissolves completely.

Remove the simple syrup from heat and let it cool to room temperature, about 20 to 30 minutes.

Cut lemons and oranges in half and juice until you have 2 cups of lemon juice and 1.5 cups of orange juice, then strain through a fine-mesh sieve.

Pour the whiskey, strained lemon juice, strained orange juice, cooled simple syrup, and 1 cup of water into a large mixing bowl or pitcher.

Whisk the mixture vigorously for about 1 minute to fully incorporate the syrup and blend the flavors.

Transfer the mixture into a shallow 9 by 13 inch freezer-safe container and place it in the freezer.

After 2 hours, remove from the freezer and use a fork to scrape and fluff the partially frozen mixture, breaking up ice crystals.

Return to the freezer and repeat the scraping process every 2 hours for the next 4 to 6 hours to achieve a granular slush texture.

Let the slush freeze undisturbed for at least 2 more hours until it reaches a thick, scoopable consistency.

When ready to serve, scoop the slush into chilled glasses, garnish with lemon or orange wheels, and serve immediately.