Place 1 lime wedge and mint leaves into a rocks glass; muddle to release oils and juice, about 10 seconds. Add sugar and 2 lime wedges; muddle to release oils and juices, about 10 seconds. Stir mixture until sugar dissolves. Add ice cubes and pour light rum and mango-flavored rum over ice; top with seltzer. Stir drink gently for 2 seconds. Garnish drink with remaining lime wedge.