Cut lemon and lime in half crosswise, and squeeze 3 tablespoons juice from each into a large zip-top plastic bag, reserving rinds. Add reserved rinds, half of mint leaves, sweetener, and pineapple juice to bag; seal tightly. Squeeze mixture 15 to 20 seconds or until oils from rinds are released and mint leaves are bruised. Refrigerate 30 minutes.
Strain mixture through a fine sieve over a pitcher, squeezing rinds to extract remaining juice. Discard fruit and mint; gently stir in tonic water.
Fill 5 tall glasses three-quarters full with crushed ice. Top ice evenly with remaining mint leaves, and pour mixture into glasses. Serve immediately