Combine sugar; water; 2/3 cup fresh pink grapefruit juice; 1 Tbsp. fresh lemon juice; and the rinds, peeled in strips, of lemons and grapefruit in a saucepan over medium-high. Bring to a boil, stirring until sugar dissolves. Remove from heat; steep 1 hour. Strain syrup; discard solids. Cool completely. Stir 5 tsp. syrup into 1 cup (or more, depending on desired flavor) seltzer water. Store syrup chilled up to 2 weeks.