To make the sour mix, place sugar cube in a glass and pour lemon juice and lime juice over top. Stir until sugar cube is completely dissolved.
To make the cocktails, fill a shaker with ice. Add the sour mix, whiskey, and Cointreau and shake until well chilled.
Fill two highball glasses with ice. Strain the contents of the shaker into the glasses, dividing between the two. Fill to the top with sparkling water.