Rub lime wedge around rims of 2 highball glasses. Dip rims into salt to coat, then fill glasses with ice.
In cocktail shaker or large glass, add 2 rosemary sprigs and agave nectar or honey; muddle gently 30 seconds. Add ¼ cup lime juice and 1 cup grapefruit juice; shake or stir vigorously 15 seconds.
Strain mixture evenly between prepared glasses (discard muddled rosemary), then top with sparkling water. Gently stir to combine.
Garnish each glass with 1 rosemary sprig and half grapefruit wheel. Serve, refrigerating any leftovers.
