In a large pitcher, combine the pineapple juice, orange juice, mango nectar, pineapple rum, and dark rum.
Refrigerate the pitcher until chilled, about 2 hours.
Stir well and then pour the rum punch into glasses filled with ice. Drizzle 1-2 tablespoons of grenadine into each glass before serving. (It should sink to the bottom.)
Garnish each drink with a pineapple wedge and some cherries.