For the ice, mix together 1 cup water, juice from 1 lime (3 tablespoons), and lime zest. Pour into ice cube tray. Float a few mint leaves in each cube and freeze.
Clean and roughly chop the rhubarb and strawberries. In a saucepan combine 1 cup water, sugar, strawberries, rhubarb, and 1 pinch kosher salt. Simmer until the rhubarb breaks down, about 10 minutes, stirring occasionally. Strain and cool. Measure out 1 ½ cups (if the syrup is not quite 1 ½ cups, add a bit of water.)
For each margarita, stir 3 ounces strawberry rhubarb syrup, 3 ounces tequila (to taste), 1 ounce lime juice, and a few dashes of orange bitters. Serve in a salt-rimmed glass over ice.