Combine half the mulberries and blueberries with the icing sugar mixture and orange juice in a saucepan over low heat. Simmer, stirring occasionally, for 5 minutes or until the mulberries soften. Remove from heat and set aside for 10 minutes to cool. Stir the remaining mulberries and blueberries into the mulberry syrup until combined.
Use a balloon whisk to whisk the yoghurt in a medium bowl until smooth and creamy. Spoon the yoghurt into a 1L (4-cup) capacity rectangular glass dish and smooth the surface.
Spoon the berries in syrup over the yoghurt and swirl with the spoon. Cover with plastic wrap and place in the fridge for 1 hour to chill.
Serve the berries in syrup on yoghurt with muesli, if desired.