Preheat the oven to 300 degrees F. Combine the lime zest and orange zest on a baking sheet. Bake for 5 minutes. Stir the zest and continue to bake until it is dry to the touch, about 5 minutes move. Remove the baking sheet from the oven and allow the zest to cool completely, about 20 minutes. In a medium bowl, combine the salt and sugar. Add the zests and stir until evenly distributed. Divide the salt mixture between 2 (4 ounce) jars with tight fitting lids. Store in a cool, dry place for up to 6 months.