Light a grill and arrange it for direct heat or preheat a grill pan. Grill the grapefruit and the limes over high heat until well browned on the bottoms, 4 to 5 minutes. Let cool slightly.
Arrange some salt on a small plate. Moisten the rims of 2 high ball glasses and coat with salt.
For each cocktail: Fill a shaker with ice and add 2 ounces of squeezed grapefruit juice, 1 ounce of squeezed lime juice, 2 ounces tequila and ½ ounce agave nectar. Close the shaker and shake well. Fill one of the prepared glasses with ice and strain the drink into the glasses. Top with 1 ounce of club soda and serve. Fill the shaker with clean ice to make the second cocktail.
note:To make simple syrup: In a saucepan, combine 1/2 cup sugar and 1/2 cup water over moderately high heat and stir until the sugar is dissolved. Let cool slightly. Measure out 1/2 cup for this recipe and reserve the rest for another use.
Recipe from epicourious!