Preheat the slow cooker if necessary; see the manufacturer's instructions. Put the nectarines and strawberries in the slow cooker pot with 50 g (2 oz) sugar, the rind of 1 orange, the juice of 1½ oranges and the measured water. Cover and cook on high for 1–1¼ hours or until the fruit is tender. Serve warm or cold.
Just before the compote is ready, mix the mascarpone with the remaining sugar, orange rind and juice. Reserve some of the amaretti biscuits for decoration. Crumble the rest with your fingertips into the bowl with the mascarpone and stir until mixed. Spoon the fruit into tumblers, top with spoonfuls of the orange mascarpone mixture and decorate with a sprinkling of amaretti biscuit crumbs.