In a cocktail shaker, muddle together the tomatoes, mint leaves, juice of half a lime, juice of half a lemon, and salt.
Pour in the simple syrup and tequila. Add ice and shake vigorously. Divide between two short glasses filled with lots of ice. Top off with club soda or sparkling water. Garnish with mint leaves, and a few grape tomatoes on a toothpick. Enjoy!