Put half the raspberries and nectarines in a food processor or blender and process until smooth.
Stir the purée and the rest of the fruit into the remaining ingredients, then transfer to a freezerproof container and freeze for 1 hour. Stir well, then return to the freezer and freeze until solid.
Serve the frozen yogurt in scoops, as you would ice cream. It will keep for up to 1 month in the freezer.