In a large bowl, sprinkle 3 tablespoons (3 packages) of unflavored gelatin over the cold water and allow the gelatin to absorb the water for 2 minutes.
Stir in the boiling water until gelatin is fully dissolved. Stir in the sugar and brown sugar until it is dissolved. Stir in the ginger ale and the rum.
Refrigerate 2 cups of the gelatin mixture until it thickens to the consistency of a gel.
Arrange strawberries in a formation in the bottom of a 6 cup mold and gently spoon the thickened gelatin into the mold over the strawberries. Refrigerate until gelatin is set but not firm. It should stick to your finger when touched so that the next layer will adhere to it but not be so soft that the next layer bleeds into it. Depending on the timing you may need to leave the gelatin in the mold out at room temperature while you prepare the next layer so that it does not become too firm before the next layer is ready.
Meanwhile, refrigerate the remaining gelatin mixture until it has thickened to the consistency of a gel. Fold in blueberries and gently spoon into mold over strawberry layer. Refrigerate until firm, about 8 hours or overnight.
To unmold, fill a sink, basin, or large bowl with warm water. Dip the mold just to the rim in the warm water for about 15 seconds. Lift from water, hold upright, and shake slightly to loosen the gelatin from mold. Place a cold, moistened plate over the top of the mold and invert the plate and mold together. Carefully lift the mold, if gelatin does not release dip the mold in the warm water and try again. Garnish with whipped topping and serve. Tip: If you don't don't have a collection of gelatin molds on hand, try some mold alternates such as bunt pans, loaf pans, metal mixing bowls, or large food storage containers. If you want to dive right in toa buy a mold, eBay is the best place to start your search.