Bring soft drink and granulated sugar to a boil in a small saucepan over medium-high; boil, undisturbed, until reduced by half, 5 to 7 minutes. Remove from heat; cool completely, about 20 minutes.

Meanwhile, zest lemons and limes (using a channel knife/citrus zester if larger zest pieces are desired); discard fruit, or reserve for another use. Dip zest into cooled soft drink syrup using a small fine mesh strainer, letting excess syrup drain from zest.

Place sanding sugar in a bowl; toss dipped zest into sanding sugar (a few at a time). Transfer sugared zest to parchment paper in a single layer; let stand until dry, about 30 minutes.