In a small pot, combine the sweetener, 1 cup water and 2 rosemary sprigs. Bring to a simmer over medium-low heat and cook until the sugar is dissolved, 5 to 8 minutes. Remove from the heat and let cool. Store in a reusable container in the refrigerator.
Fill 2 glasses with ice. In a cocktail shaker, combine 1/4 cup of the rosemary simple syrup with the cranberry juice (reserve the remaining simple syrup for another use). Shake and pour over the ice into the glasses. Top with the seltzer. Garnish each with a rosemary sprig.
You can add more simple syrup if you like a sweeter mocktail.