Chill all ingredients, except bananas. Thinly slice 6 bananas. Place banana slices, lime juice and sugar in a blender. Blend for a few seconds, until mixture is smooth. Pour over block of ice in large punch bowl. Pour in light rum, dark rum, pineapple juice and mango nectar. Stir. Refrigerate for one hour. Cut remaining bananas and limes into thin slices and float on top of punch.
Serve in a Punch Bowl