In a cocktail shaker, combine the light rum, dark rum, triple sec, passion fruit juice, pineapple juice, lemon juice, lime juice, grenadine, and bitters.
Fill the shaker with ice and shake vigorously until well chilled.
Strain the mixture into a tall glass filled with fresh ice.
If using, carefully float the overproof rum on top by pouring it over the back of a spoon.
Garnish with a mint sprig and a slice of your favorite tropical fruit.