Add the mint to a cocktail shaker and muddle it gently. Add the watermelon cubes and use the muddler to release all of the juice. (If you’re using watermelon juice, simply muddle the mint first then add the juice in Step 2.)
Add the vodka, rum, lime juice and simple syrup. Fill with ice and shake until cold.
Fill a lowball glass with the crushed ice, then strain in the drink. Garnish with additional watermelon slices and mint.
Notes
*To convert to tablespoons, 1 ounce = 2 tablespoons