Place the orange and grapefruit pieces in a medium bowl and set aside. In a medium saucepan over medium heat, combine the sugar and marmalade and cook until sugar is dissolved, stirring often. Turn off the heat and add the orange soda and grape juice; cool to room temperature. Pour the dressing over the citrus segments in the bowl; cover and refrigerate overnight.
To serve, use a slotted spoon to divide the fruit into four individual bowls. Garnish each serving generously with toasted coconut and almond slivers.